Start sketching out a PC compatible machine.

[TomHarte]

Primarily to get x86 code into continuous integration:

• establish the PC memory map;
• have an emulated 8088 stepping through instructions — full interpretation will do for now; and
• note port IO for further development.

Also:

• fixes an issue in which byte-length port numbers were sign extended;
• reduces continuous integration from running on every push and for every pull request to only the latter; and
• includes a very yucky workaround for GCC's failure to use the explicit instantiations at the bottom of Decoder.cpp. I can't figure out what error I've made and temporarily have only continuous integration to test fixes. So I'm putting a pin in that until I next have access to a local copy of GCC.

The longer-term arc involves an 80286, cached decoding, etc.