Adding docs to macros and fixeing rust lint warnings

[dr-orlovsky]

New versions of rust compiler fails to build the crate due to the absent docs for some macros. They also produce some lint warning. This PR fixes the problem.

Sample of the failed build before the fixes applied: https://travis-ci.org/rust-bitcoin/bitcoin_hashes/builds/589871443?utm_source=github_status&utm_medium=notification

UPDATE: Had to roll back some of the linting improvements which were not supported by the current stable rustc version

[stevenroose]

LGTM

[dr-orlovsky]

Can we merge this thing pls to unlock #63 which is it's dependence?

[apoelstra]

Why is the diff so big? Is there a purpose to reordering these methods to no longer be in alphabetical order?

[dr-orlovsky]

1. The reason of method reordering is that compiler asks (via warning) that the order of the methods has to be the same like in the trait specification.
2. The size of the diff: it's not big, it just contains the changes requested by the compiler warnings, i.e. new single-line doc-style comments for the macros that had not them + re-ordering of the impl methods to the order used in the trait declarations.

[dr-orlovsky]

This PR is blocking #63, required for further processing with hash_newtypes, which are nearly done for rust-bitcoin: rust-bitcoin/rust-bitcoin#349

[stevenroose]

My compiler doesn't complain about the order of methods. I guess if you would limit the PR to the doc and not change the order, it could get merged without any hassle..

cargo 1.39.0 (1c6ec66d5 2019-09-30)

[dr-orlovsky]

Done