Implement str.count and add tests for it.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.
Fix issues in str.count implementation.

See pull request #343.
Merge pull request #344 from dhylands/stmhal-systick-cleanup

Cleanup systick for stmhal
Merge pull request #343 from xbe/master

Implement str.count and add tests for it.
py: In string.count, handle case of zero-length needle.
stmhal - More systick cleanup. Fix HAL_Delay