It has a single call site.

It's a macro with four clauses, three of which are doing one thing, and
the fourth is doing something completely different. This commit splits
it into two macros, which is more sensible.

It currently processes `ACTIVE_FEATURES` separately from
`ACCEPTED_FEATURES`, `REMOVED_FEATURES`, and `STABLE_REMOVED_FEATURES`,
for no good reason. This commit treats them uniformly.

I found this function very confusing, and it took me quite some time to
work out what it was doing. These comments capture that hard-earned
knowledge.

It has a single call site. This increases consistency because other
errors within `features` are emitted directly.

There is a single features (`no_stack_check`) in
`STABLE_REMOVED_FEATURES`. But the treatment of
`STABLE_REMOVED_FEATURES` and `REMOVED_FEATURES` is actually identical.
So this commit just merges them, and uses a comment to record
`no_stack_check`'s unique "stable removed" status.

This also lets `State::Stabilized` (which was a terrible name) be
removed.

The new way of doing things:
- Avoids some code duplication.
- Distinguishes the `crate_edition` (which comes from `--edition`) and
  the `features_edition` (which combines `--edition` along with any
  `rustc_20XX_preview` features), which is useful.
- Has a simpler initial loop, one that just looks for
  `rustc_20XX_preview` features in order to compute `features_edition`.
- Creates a fallible alternative to `Features::enabled`, which is
  useful.

It's not easy to see how exactly the old and new code are equivalent,
but it's reassuring to know that the test coverage is quite good for
this stuff.

The word "active" is currently used in two different and confusing ways:
- `ACTIVE_FEATURES` actually means "available unstable features"
- `Features::active_features` actually means "features declared in the
  crate's code", which can include feature within `ACTIVE_FEATURES` but
  also others.

(This is also distinct from "enabled" features which includes declared
features but also some edition-specific features automatically enabled
depending on the edition in use.)

This commit changes the `Features::active_features` to
`Features::declared_features` which actually matches its meaning.
Likewise, `Features::active` becomes `Features::declared`.

For the reasons described in the previous commit.

Currently `rust_20XX_preview` features aren't recorded as declared even
when they are explicit declared. Similarly, redundant edition-dependent
features (e.g. `test_2018_feature`) aren't recorded as declared.

This commit marks them as recorded. There is no detectable functional
change, but it makes things more consistent.