add parity support

[nerdralph]

MCUdude/MicroCore#101 (comment)

[nerdralph]

Adding parity support is not difficult to add, but it is difficult to do in a simple and efficient way that does not add overhead to the code when parity is not used.
The current inline asm code is already complex, and I would prefer to avoid adding #ifdef statements to the code for conditional compilation of parity.

One possibility is to use metaprogramming, with common code blocks that are assembled by a makefile, to generate separate include files for a parity and non-parity version of picoUART.