There shouldn't be any functional changes here.

* Some doc comments.
* Construct `FixArgs` at once so it doesn't need to bother with unnecessary Options.
* Remove IdiomEditionMismatch, it is not used.
* Use a general deduping mechanism for fix messages.

What was previously "Fixing" was a message for after the fixes had
been applied. I think it would be clearer if it said "Fixed",
to indicate that the fixes had actually finished.

The new "Fixing" is posted just before it starts. This is verbose-only
since it is a little noisy.

This helps indicate which edition you are moving from and to.

This is intended to help make it easier to test the 2021 edition.

* `--edition` always means "next" edition.
* `--edition` when on the most recent edition is not an error, just a warning.
* Support fix to 2021 edition.

# Conflicts:
#	src/cargo/ops/fix.rs
#	src/doc/src/reference/unstable.md

This attempts to centralize all the edition stuff in one place (the
`Edition` enum) so that adding a new edition or stabilizing one should
be relatively little work (and more importantly, we don't miss things).

Importantly, this changes `cargo new` to default to the latest stable.
It also changes the `cargo fix --edition-idiom` behavior to only apply
idioms for the *current* edition.

This was deprecated, never officially part of the stable release.

This fixes an issue where warnings would be printed for packages
migrating to 2021 in a workspace (that the "resolver" field is ignored,
which is wrong).

This also places the default resolver logic in one place, and should
make it easier to update later.