Rewrite MMTk-specific code with WHEN_USING_MMTK

When the "#if USE_MMTK" directive and the `if (rb_mmtk_enabled_p())`
statement are used together, it will be very complicated and confusing.
We rewrite `string.c` using the convenient `WHEN_USING_MMTK`,
`WHEN_NOT_USING_MMTK` and `WHEN_USING_MMTK2` macros when possible.

This commit does not change the semantics of the existing code in
`string.c`.

Author: wks

Diff for: internal/mmtk_macros.h

@@ -5,13 +5,17 @@
 
 #if USE_MMTK
 #define IF_USE_MMTK(a) a
+#define IF_NOT_USE_MMTK(a)
 #define IF_USE_MMTK2(a, b) a
 #define WHEN_USING_MMTK(a) if (rb_mmtk_enabled_p()) { a }
+#define WHEN_NOT_USING_MMTK(a) if (!rb_mmtk_enabled_p()) { a }
 #define WHEN_USING_MMTK2(a, b) if (rb_mmtk_enabled_p()) { a } else { b }
 #else
 #define IF_USE_MMTK(a)
+#define IF_NOT_USE_MMTK(a) a
 #define IF_USE_MMTK2(a, b) b
 #define WHEN_USING_MMTK(a)
+#define WHEN_NOT_USING_MMTK(a) a
 #define WHEN_USING_MMTK2(a, b) b
 #endif