update snappy version & merge all prs still waiting on external feedback

[dch]

• closes #27

• closes #29

• closes #33 with thanks @skaes

• [x] check on arm64 @dch

• [x] check on OTP23 @dch

• [x] check on OTP 21 @dch

• [x] check on OTP 20 @dch

• [ ] confirm from @skaes

[dch]

This took several attempts to get versions of enc & rebar3 that are OTP20+ compatible, and compile on newer versions as well. LGTM finally.