This gives a functioning, independent MSC device.

Make str and bytes be proper types

This is pre-requisite for having efficient implementation of str<->bytes
conversion, and having that efficient is required with unfortunare
str vs bytes dichotomy in Python3.

Implement str.partition and add tests for it.

Switch str.rpartition to search from left to right. Factor the
duplicate code into one helper function.

Some math functions

Just low-level read/write support.  No filesystem yet.

stmhal - Add usart support