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.