[fastx types] make size of Move's `address` type configurable

[sblackshear]

Move account addresses are 16 bytes. We would like to make them the same size as fastX authenticators (i.e., FastPayAddress), and (ideally) fastX ObjectID's too. We haven't decided on the size of either of those types yet, but they will likely need to be >= 20 bytes for safety (e.g., a 32 byte hash truncated to the shortest length that is safe w.r.t collisions).

Whatever length(s) we chose, we need to be able to adjust Move addresses accordingly. The size of Move addresses is determined by this constant in the Diem codebase. We need to tweak it via one mechanism or another. Some ideas:

• cargo magic to set a constant at compile-time via a build config (preferred long-term solution)
• Use cargo's patch feature to override the address length locally (probably the quickest short term solution)
• Use a fork of Diem with our preferred address length (least preferred solution)

[lxfind]

This is landed in Move repo: diem/move#10

[lxfind]

Fully switched to 20-bytes ObjectID in #262 #263 #264