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/objint_mpz.c b/py/objint_mpz.c
index 9cdbb71..516fb52 100644
--- a/py/objint_mpz.c
+++ b/py/objint_mpz.c
@@ -118,7 +118,7 @@
         zrhs = &z_int;
     } else if (MP_OBJ_IS_TYPE(rhs_in, &mp_type_int)) {
         zrhs = &((mp_obj_int_t*)rhs_in)->mpz;
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
     } else if (MP_OBJ_IS_TYPE(rhs_in, &mp_type_float)) {
         return mp_obj_float_binary_op(op, mpz_as_float(zlhs), rhs_in);
     } else if (MP_OBJ_IS_TYPE(rhs_in, &mp_type_complex)) {
@@ -130,7 +130,7 @@
     }
 
     if (0) {
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
     } else if (op == MP_BINARY_OP_TRUE_DIVIDE || op == MP_BINARY_OP_INPLACE_TRUE_DIVIDE) {
         mp_float_t flhs = mpz_as_float(zlhs);
         mp_float_t frhs = mpz_as_float(zrhs);
@@ -292,7 +292,7 @@
     }
 }
 
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
 mp_float_t mp_obj_int_as_float(mp_obj_t self_in) {
     if (MP_OBJ_IS_SMALL_INT(self_in)) {
         return MP_OBJ_SMALL_INT_VALUE(self_in);