More error correction preparation (upgrades to `Field`, `Polynomial`, `InvalidResidueError` and others)

[apoelstra]

This PR has a fairly large diff but the commits should be self-contained and much of the diff consists of comments. In particular this introduces a new type FieldVec to back Polynomial, which strives to provide a sensible API even in a no-alloc case:

• With an allocator, arbitrarily-large vectors (and therefore arbitrarily large checksums) are supported.
• Without an allocator, the FieldVec type panics for large checksums, but since it is only (planned to be) used in error correction, it should be easy to detect this up front and provide a sensible error to the user.

The threshold for "large" is currently set at "bigger than bech32", but it is easy to change this and even easy to add a feature-gate.

I believe this completes all the changes to the traits needed for error correction, and the next PR can just implement correction.

[apoelstra]

CI failed because I had a doccomment referring to a field of Checksum that didn't yet exist. Backported the commit that introduced it.

But now this PR is way too big :). Gonna close it and split it.