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/py/objfloat.c b/py/objfloat.c
index 6732f31..b608b1a 100644
--- a/py/objfloat.c
+++ b/py/objfloat.c
@@ -39,7 +39,7 @@
 #include "runtime0.h"
 #include "runtime.h"
 
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
 
 #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
 #include "formatfloat.h"
@@ -170,4 +170,4 @@
     return mp_obj_new_float(lhs_val);
 }
 
-#endif // MICROPY_ENABLE_FLOAT
+#endif // MICROPY_PY_BUILTINS_FLOAT