[ruby/prism] Remove attr_writer's for ParseResult#start_line and #offsets

* As the user should not set these.
* Use #instance_variable_set/rb_iv_set() instead internally.

https://github.com/ruby/prism/commit/cace09fb8c

lib/prism/parse_result.rb

@@ -9,10 +9,10 @@ module Prism
     attr_reader :source
 
     # The line number where this source starts.
-    attr_accessor :start_line
+    attr_reader :start_line
 
     # The list of newline byte offsets in the source code.
-    attr_accessor :offsets
+    attr_reader :offsets
 
     # Create a new source object with the given source code.
     def initialize(source)

prism/templates/ext/prism/api_node.c.erb

@@ -49,13 +49,13 @@ pm_source_new(pm_parser_t *parser, rb_encoding *encoding) {
 
 void
 pm_source_init(VALUE source, pm_parser_t *parser) {
-    rb_funcall(source, rb_intern("start_line="), 1, LONG2NUM(parser->start_line));
+    rb_iv_set(source, "@start_line", LONG2NUM(parser->start_line));
 
     VALUE offsets = rb_ary_new_capa(parser->newline_list.size);
     for (size_t index = 0; index < parser->newline_list.size; index++) {
         rb_ary_push(offsets, ULONG2NUM(parser->newline_list.offsets[index]));
     }
-    rb_funcall(source, rb_intern("offsets="), 1, offsets);
+    rb_iv_set(source, "@offsets", offsets);
 }
 
 typedef struct pm_node_stack_node {

prism/templates/lib/prism/serialize.rb.erb

@@ -79,11 +79,11 @@ module Prism
       end
 
       def load_start_line
-        source.start_line = load_varsint
+        source.instance_variable_set :@start_line, load_varsint
       end
 
       def load_line_offsets
-        source.offsets = load_varuint.times.map { load_varuint }
+        source.instance_variable_set :@offsets, load_varuint.times.map { load_varuint }
       end
 
       def load_comments