support builtin features with Ruby and C.

Support loading builtin features written in Ruby, which implement
with C builtin functions.
[Feature #16254]

Several features:

(1) Load .rb file at boottime with native binary.

Now, prelude.rb is loaded at boottime. However, this file is contained
into the interpreter as a text format and we need to compile it.
This patch contains a feature to load from binary format.

(2) __builtin_func() in Ruby call func() written in C.

In Ruby file, we can write `__builtin_func()` like method call.
However this is not a method call, but special syntax to call
a function `func()` written in C. C functions should be defined
in a file (same compile unit) which load this .rb file.

Functions (`func` in above example) should be defined with
  (a) 1st parameter: rb_execution_context_t *ec
  (b) rest parameters (0 to 15).
  (c) VALUE return type.
This is very similar requirements for functions used by
rb_define_method(), however `rb_execution_context_t *ec`
is new requirement.

(3) automatic C code generation from .rb files.

tool/mk_builtin_loader.rb creates a C code to load .rb files
needed by miniruby and ruby command. This script is run by
BASERUBY, so *.rb should be written in BASERUBY compatbile
syntax. This script load a .rb file and find all of __builtin_
prefix method calls, and generate a part of C code to export
functions.

tool/mk_builtin_binary.rb creates a C code which contains
binary compiled Ruby files needed by ruby command.

builtin.c

@@ -0,0 +1,40 @@
+#include "internal.h"
+#include "vm_core.h"
+#include "iseq.h"
+#include "builtin.h"
+
+#include "builtin_binary.inc"
+
+static const unsigned char*
+builtin_lookup(const char *feature, size_t *psize)
+{
+    for (int i=0; i<BUILTIN_BINARY_SIZE; i++) {
+        if (strcmp(builtin_binary[i].feature, feature) == 0) {
+            *psize = builtin_binary[i].bin_size;
+            return builtin_binary[i].bin;
+        }
+    }
+    rb_bug("builtin_lookup: can not find %s\n", feature);
+}
+
+void
+rb_load_with_builtin_functions(const char *feature_name, const char *fname, const struct rb_builtin_function *table)
+{
+    // search binary
+    size_t size;
+    const unsigned char *bin = builtin_lookup(feature_name, &size);
+
+    // load binary
+    GET_VM()->builtin_function_table = table;
+    const rb_iseq_t *iseq = rb_iseq_ibf_load_cstr((const char *)bin, size);
+    GET_VM()->builtin_function_table = NULL;
+
+    // exec
+    rb_iseq_eval(iseq);
+}
+
+void
+Init_builtin(void)
+{
+    //
+}