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/builtin.c b/py/builtin.c
index 178a683..c336378 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -37,7 +37,7 @@
 #include "runtime.h"
 #include "builtin.h"
 
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
 #include <math.h>
 #endif
 
@@ -104,7 +104,7 @@
             val = -val;
         }
         return MP_OBJ_NEW_SMALL_INT(val);
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
     } else if (MP_OBJ_IS_TYPE(o_in, &mp_type_float)) {
         mp_float_t value = mp_obj_float_get(o_in);
         // TODO check for NaN etc