Rename bultins config variables to MICROPY_PY_BUILTINS_*.

This renames:
MICROPY_PY_FROZENSET -> MICROPY_PY_BUILTINS_FROZENSET
MICROPY_PY_PROPERTY -> MICROPY_PY_BUILTINS_PROPERTY
MICROPY_PY_SLICE -> MICROPY_PY_BUILTINS_SLICE
MICROPY_ENABLE_FLOAT -> MICROPY_PY_BUILTINS_FLOAT

See issue #35 for discussion.
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index c56c9fa..1b1d938 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -41,7 +41,7 @@
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (1)
-#define MICROPY_PY_FROZENSET        (1)
+#define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
 #define MICROPY_PY_CMATH            (1)