Combined efforts on AVR mocks

[ianfixes]

Don't merge yet.

Need to address this list of issues (which may increase at some future point, depending on how things unfold)

• Add unittest compilation tests for libraries that include only parts of Arduino #118
• Create cpp/avr/ or cpp/avr_includes/avr as appropriate to support <avr/*.h> #119
• Create 16/32-bit compatible AVR Registers mocks, _SFR_MEM8 in particular #120
• Establish mocks for interrupt.h #121

Highlights from CHANGELOG.md

• See CHANGELOG.md for more

[hlovdal]

Updated (relative) links:

• Add unittest compilation tests for libraries that include only parts of Arduino #118
• Create cpp/avr/ or cpp/avr_includes/avr as appropriate to support <avr/*.h> #119
• Create 16/32-bit compatible AVR Registers mocks, _SFR_MEM8 in particular #120
• Establish mocks for interrupt.h #121

[ianfixes]

@PRosenb and @hlovdal what's the priority on getting this merged? I will have to resolve some conflicts, and really I'm getting back up to speed on where this was a year ago.