Tracking: Threading + IO redesign

This is a tracking issue for the redesign work for how we model protocol execution and IO.

Tasks

[x] Introduce mpz-common which provides types and traits for threading and synchronization. It also adds serio for our IO abstraction. #107
[x] Encapsulate our coin-toss protocols async layer #108
[x] Update mpz-ot to use mpz-common and deprecate the KOS15 actor impl #109
[x] Finish implementing OT traits for KOS15 shared variant.
[x] Implement a multi-threaded executor/context in mpz-common
[x] Update mpz-garble
[x] Update mpz-share-conversion