Immediately go to native for RSTRING_PTR()

* That is, use RSTRING_PTR() as a hint the C extension is going to access
  the byte with native memory accesses.
* Adapt RSTRING_PTR() spec since it always goes native directly now.
* 64s => 59s for a small `bundle install`.

(cherry picked from commit 9a22abd)

Author: eregon

spec/truffle/capi/string_spec.rb

@@ -15,10 +15,10 @@
     @s = CApiTruffleStringSpecs.new
   end
 
-  it "does not store the String to native memory if not needed" do
+  it "stores the String to native memory even if not needed for efficiency" do
     str = "foobar"
     @s.string_ptr(str).should == "f"
-    Truffle::CExt.native_string?(str).should == false
+    Truffle::CExt.native_string?(str).should == true
   end
 
   it "stores the String to native memory if the address is returned" do

src/main/c/cext/ruby.c

@@ -812,11 +812,7 @@ char *rb_string_value_cstr(VALUE *value_pointer) {
 }
 
 char *RSTRING_PTR_IMPL(VALUE string) {
-  if (polyglot_as_boolean(RUBY_CEXT_INVOKE_NO_WRAP("native_string?", string))) {
-    return NATIVE_RSTRING_PTR(string);
-  } else {
-    return (char *)polyglot_as_i8_array(RUBY_CEXT_INVOKE_NO_WRAP("RSTRING_PTR", string));
-  }
+  return NATIVE_RSTRING_PTR(string);
 }
 
 char *RSTRING_END(VALUE string) {