Refactor parser_params by removing "in_main" flag

The relation between parser_param#base_block and #in_main were very
subtle.
A main script (that is passed via a command line) was parsed under
base_block = TOPLEVEL_BINDING and in_main = 1.
A script loaded by Kernel#require was parsed under
base_block = NULL and in_main = 0.
If base_block is non-NULL and in_main == 0, it is parsed by Kernel#eval
or family.

However, we know that TOPLEVEL_BINDING has no local variables when a
main script is parsed.  So, we don't have to parse a main script under
base_block = TOPLEVEL_BINDING.

Instead, this change parses a main script under base_block = 0.
If base_block is non-NULL, it is parsed by Kernel#eval or family.
By this simplication, "in_main" is no longer needed.

parse.y

@@ -270,7 +270,6 @@ struct parser_params {
     unsigned int debug: 1;
     unsigned int has_shebang: 1;
     unsigned int in_defined: 1;
-    unsigned int in_main: 1;
     unsigned int in_kwarg: 1;
     unsigned int in_def: 1;
     unsigned int in_class: 1;
@@ -330,7 +329,7 @@ static int parser_yyerror(struct parser_params*, const YYLTYPE *yylloc, const ch
 #ifdef RIPPER
 #define compile_for_eval	(0)
 #else
-#define compile_for_eval	(p->base_block != 0 && !p->in_main)
+#define compile_for_eval	(p->base_block != 0)
 #endif
 
 #define token_column		((int)(p->lex.ptok - p->lex.pbeg))
@@ -11649,7 +11648,7 @@ static void
 local_push(struct parser_params *p, int toplevel_scope)
 {
     struct local_vars *local;
-    int inherits_dvars = toplevel_scope && (compile_for_eval || p->in_main /* is p->in_main really needed? */);
+    int inherits_dvars = toplevel_scope && compile_for_eval;
     int warn_unused_vars = RTEST(ruby_verbose);
 
     local = ALLOC(struct local_vars);
@@ -12309,7 +12308,6 @@ rb_parser_set_context(VALUE vparser, const struct rb_block *base, int main)
     TypedData_Get_Struct(vparser, struct parser_params, &parser_data_type, p);
     p->error_buffer = main ? Qfalse : Qnil;
     p->base_block = base;
-    p->in_main = main;
     return vparser;
 }
 #endif

ruby.c

@@ -1562,8 +1562,6 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
     const char *s;
     char fbuf[MAXPATHLEN];
     int i = (int)proc_options(argc, argv, opt, 0);
-    rb_binding_t *toplevel_binding;
-    const struct rb_block *base_block;
     unsigned int dump = opt->dump & dump_exit_bits;
 
     if (opt->dump & (DUMP_BIT(usage)|DUMP_BIT(help))) {
@@ -1760,13 +1758,7 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
     ruby_set_argv(argc, argv);
     process_sflag(&opt->sflag);
 
-    GetBindingPtr(rb_const_get(rb_cObject, rb_intern("TOPLEVEL_BINDING")),
-		  toplevel_binding);
-    /* need to acquire env from toplevel_binding each time, since it
-     * may update after eval() */
-
-    base_block = toplevel_context(toplevel_binding);
-    rb_parser_set_context(parser, base_block, TRUE);
+    rb_parser_set_context(parser, 0, TRUE);
 
     if (opt->e_script) {
 	VALUE progname = rb_progname;
@@ -1867,7 +1859,11 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 		rb_enc_copy(path, opt->script_name);
 	    }
 	}
-	base_block = toplevel_context(toplevel_binding);
+
+        rb_binding_t *toplevel_binding;
+        GetBindingPtr(rb_const_get(rb_cObject, rb_intern("TOPLEVEL_BINDING")),
+                      toplevel_binding);
+        const struct rb_block *base_block = toplevel_context(toplevel_binding);
 	iseq = rb_iseq_new_main(&ast->body, opt->script_name, path, vm_block_iseq(base_block));
 	rb_ast_dispose(ast);
     }