encode_unsigned_1

[mlwilkerson]

closes #671

• [x] test boundary case for smallest bigint
• [x] add proptest for non-integer
• [x] add proptest for negative integer
• [x] fix bug with dropping middle zeros
• [x] fix bug with BigInt with trailing zeros
• [x] migrate unit tests to integration tests

[mlwilkerson]

I've migrated the tests to the integration testing pattern. Should the unit tests be removed entirely? That'd result in a loss of some coverage from the proptests, at least.

Also, when doing these as integration tests, all of the ones that use big integer inputs are failing with badarg when I wouldn't expect them to--and they don't fail in the unit tests. This suggests that something higher up in the stack is balking at these big integers, but I haven't investigated that.

[bcardarella]

FYI I'll ask about this PR in today's standup