py/objcode: Factor code object out into its own file.

The `mp_obj_code_t` and `mp_type_code` code object was defined internally
in both `py/builtinevex.c` and `py/profile.c`, with completely different
implementations (the former very minimal, the latter quite complete).

This commit factors these implementations into a new, separate source file,
and allows the code object to have four different modes, selected at
compile-time:

- MICROPY_PY_BUILTINS_CODE_NONE: code object not included in the build.

- MICROPY_PY_BUILTINS_CODE_MINIMUM: very simple code object that just holds
  a reference to the function that it represents.  This level is used when
  MICROPY_PY_BUILTINS_COMPILE is enabled.

- MICROPY_PY_BUILTINS_CODE_BASIC: simple code object that holds a reference
  to the proto-function and its constants.

- MICROPY_PY_BUILTINS_CODE_FULL: almost complete implementation of the code
  object.  This level is used when MICROPY_PY_SYS_SETTRACE is enabled.

Signed-off-by: Damien George <damien@micropython.org>

py/py.cmake

@@ -72,6 +72,7 @@ set(MICROPY_SOURCE_PY
     ${MICROPY_PY_DIR}/objattrtuple.c
     ${MICROPY_PY_DIR}/objbool.c
     ${MICROPY_PY_DIR}/objboundmeth.c
+    ${MICROPY_PY_DIR}/objcode.c
     ${MICROPY_PY_DIR}/objcell.c
     ${MICROPY_PY_DIR}/objclosure.c
     ${MICROPY_PY_DIR}/objcomplex.c