Implement sys.settrace as the lowest level of a debugger or profiler

[tannewt]

sys.settrace could be a very useful tool for performance analysis via python rather than requiring a lot from the VM/C itself.

There is a PR to micropython for it here: micropython#4265 from @malinah

[bsd5]

The above ticket was superseded in micropython#5026 and merged :)

[dhalbert]

MICROPY_PY_SYS_SETTRACE is now available, but we don't turn it on. But requires MICROPY_PERSISTENT_CODE_SAVE to be 1 and MICROPY_COMP_CONST to be 0.