py: Add MICROPY_ENABLE_COMPILER and MICROPY_PY_BUILTINS_EVAL_EXEC opts.

MICROPY_ENABLE_COMPILER can be used to enable/disable the entire compiler,
which is useful when only loading of pre-compiled bytecode is supported.
It is enabled by default.

MICROPY_PY_BUILTINS_EVAL_EXEC controls support of eval and exec builtin
functions.  By default they are only included if MICROPY_ENABLE_COMPILER
is enabled.

Disabling both options saves about 40k of code size on 32-bit x86.

py/parse.c

@@ -38,6 +38,8 @@
 #include "py/runtime.h"
 #include "py/builtin.h"
 
+#if MICROPY_ENABLE_COMPILER
+
 #define RULE_ACT_ARG_MASK       (0x0f)
 #define RULE_ACT_KIND_MASK      (0x30)
 #define RULE_ACT_ALLOW_IDENT    (0x40)
@@ -1079,3 +1081,5 @@ void mp_parse_tree_clear(mp_parse_tree_t *tree) {
         chunk = next;
     }
 }
+
+#endif // MICROPY_ENABLE_COMPILER