Grep for TODO

[tcharding]

Consider adding a tool that greps for TODO (excluding changelog) as part of pre-release checks, at least for rust-bitcoin we should have no todo's in the codebase. I personally use them for questions and todos during dev that should not be merged but they slip through into master occasionally and should definitely not be released.

Note in point:

cd path/to/rust-bitcoin
git checkout 85157078
git grep TODO
bitcoin/CHANGELOG.md:1:  # 0.32.0 - TODO: Add date when we do final release
bitcoin/src/consensus/mod.rs:151:            Other(_) => None, // TODO: Is this correct?

[apoelstra]

Also FIXME.