https://github.com/riscv/riscv-isa-manual/commit/4dcaa944ba40e074d25516a157fc37f7491b71cc

Resolves #93

https://github.com/riscv/riscv-isa-manual/commit/f2ed45b1791bb602657adc2ea9ab5fc409c62542

https://github.com/riscv/riscv-isa-manual/issues/10

This improves simulator perf when a thread is idle, or waiting on HTIF.

https://github.com/riscv/riscv-isa-manual/issues/14

https://github.com/riscv/riscv-isa-manual/issues/4

Also, refactor gdbserver code to not duplicate VM decoding logic.

Gdb protocol fixes

Resolves #76

Resolves #88.

h/t Ron Minnich

See https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/JV-Hj3W5Kw8