[ruby/prism] String literal hash keys should be frozen

String literal hash keys can't be mutated by the user so we should mark
them as frozen. We were seeing instructions for hashes with string
literal keys using two `putstring` instructions when it should be a
`putobject` and `putstring`.

Code example:

```ruby
{ "a" => "b" }
```

Instructions before:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject                              "a"                       (   2)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putstring                              "a"                       (   1)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
```

Instructions after:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject                              "a"                       (   2)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putobject                              "a"                       (   1)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
```

https://github.com/ruby/prism/commit/b14ae55385

prism/prism.c

@@ -1340,6 +1340,11 @@ pm_assoc_node_create(pm_parser_t *parser, pm_node_t *key, const pm_token_t *oper
         flags = key->flags & value->flags & PM_NODE_FLAG_STATIC_LITERAL;
     }
 
+    // Hash string keys should be frozen
+    if (PM_NODE_TYPE_P(key, PM_STRING_NODE)) {
+        key->flags |= PM_STRING_FLAGS_FROZEN;
+    }
+
     *node = (pm_assoc_node_t) {
         {
             .type = PM_ASSOC_NODE,

test/prism/snapshots/unparser/corpus/literal/literal.txt

@@ -9,7 +9,7 @@
         │   │   ├── @ AssocNode (location: (1,2)-(1,21))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (1,2)-(1,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (1,2)-(1,3) = "\""
         │   │   │   │   ├── content_loc: (1,3)-(1,6) = "foo"
         │   │   │   │   ├── closing_loc: (1,6)-(1,7) = "\""
@@ -39,7 +39,7 @@
         │   │   └── @ AssocNode (location: (1,23)-(1,36))
         │   │       ├── key:
         │   │       │   @ StringNode (location: (1,23)-(1,28))
-        │   │       │   ├── flags: ∅
+        │   │       │   ├── flags: frozen
         │   │       │   ├── opening_loc: (1,23)-(1,24) = "\""
         │   │       │   ├── content_loc: (1,24)-(1,27) = "bar"
         │   │       │   ├── closing_loc: (1,27)-(1,28) = "\""
@@ -59,7 +59,7 @@
         │   │   ├── @ AssocNode (location: (4,2)-(4,14))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (4,2)-(4,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (4,2)-(4,3) = "\""
         │   │   │   │   ├── content_loc: (4,3)-(4,6) = "foo"
         │   │   │   │   ├── closing_loc: (4,6)-(4,7) = "\""
@@ -75,7 +75,7 @@
         │   │   └── @ AssocNode (location: (4,16)-(4,29))
         │   │       ├── key:
         │   │       │   @ StringNode (location: (4,16)-(4,21))
-        │   │       │   ├── flags: ∅
+        │   │       │   ├── flags: frozen
         │   │       │   ├── opening_loc: (4,16)-(4,17) = "\""
         │   │       │   ├── content_loc: (4,17)-(4,20) = "bar"
         │   │       │   ├── closing_loc: (4,20)-(4,21) = "\""
@@ -184,7 +184,7 @@
         │   │   ├── @ AssocNode (location: (10,2)-(10,21))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (10,2)-(10,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (10,2)-(10,3) = "\""
         │   │   │   │   ├── content_loc: (10,3)-(10,6) = "foo"
         │   │   │   │   ├── closing_loc: (10,6)-(10,7) = "\""
@@ -231,7 +231,7 @@
         │   │   ├── @ AssocNode (location: (13,2)-(13,14))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (13,2)-(13,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (13,2)-(13,3) = "\""
         │   │   │   │   ├── content_loc: (13,3)-(13,6) = "foo"
         │   │   │   │   ├── closing_loc: (13,6)-(13,7) = "\""

test/prism/snapshots/unparser/corpus/literal/send.txt

@@ -1310,7 +1310,7 @@
         │   │               └── @ AssocNode (location: (64,13)-(64,25))
         │   │                   ├── key:
         │   │                   │   @ StringNode (location: (64,13)-(64,18))
-        │   │                   │   ├── flags: ∅
+        │   │                   │   ├── flags: frozen
         │   │                   │   ├── opening_loc: (64,13)-(64,14) = "\""
         │   │                   │   ├── content_loc: (64,14)-(64,17) = "baz"
         │   │                   │   ├── closing_loc: (64,17)-(64,18) = "\""