Fix two bugs related to incomplete read/write, and clippy tidying

Updated my toolchain to nightly rustc 1.57.0 (f1edd0429 2021-11-29) (from rustc 1.56.1 (59eed8a2a 2021-11-01))

mentioned in merge request !250 (merged)

requested review from @nickm

Looks okay modulo those two issues.

mentioned in commit Diziet/arti@6c5e099b

mentioned in commit Diziet/arti@9a77b23c

added 2 commits

• 6c5e099b - handshake: Use read_exact, not read and checking len
• 9a77b23c - clippy: Rename a `decode_chanmsg` from `handle_`

Compare with previous version

resolved all threads

Fixed, thanks. I haven't squashed, mostly to avoid having to rebase !250 (merged) (and because the intermediate states of the tree don't seem terrible). LMK if you squash and I'll sort out !250 (merged) then.

LGTM; will merge as-is.

merged