std: fix Cell's Show instance.

Previously it was printing the address of the Unsafe contained in the
Cell (i.e. the address of the Cell itself). This is clearly useless, and
was presumably a mistake due to writing `*&` instead of `&*`.

However, this later expression is likely also incorrect, since it takes
a reference into a Cell while other user code is executing (i.e. the
Show instance for the contained type), hence the contents should just be
copied out.

Use memmem when searching for usages in ide-db

We already have this dependency, so there is no reason not to use it as it is generally faster than std in our use case.

Replace `rustc_semver` with `rustc_session::RustcVersion`

Allows dropping a dependency but there is a behaviour change here, the following versions are no longer accepted:

* `1` (would need to be `1.0` or `1.0.0`)
* `1.0.0-alpha`
* `1.0.0-alpha.2`
* `1.0.0-beta`

But this seems unlikely to effect anybody, I did some GitHub searching and found no occurrences outside our UI tests

r? `@flip1995`

changelog: none