Prepared most `StrExt` pattern using methods for stabilization
Made iterator-returning methods return newtypes
Adjusted some docs to be forwards compatible with a generic pattern API
mk: Stop generating docs for deprecated crates

These crates are all deprecated for their rust-lang/$crate equivalents and by
generating docs we're generating broken links. The documentation for these
crates are generated out-of-tree and are managed separately, so we're not losing
the documentation altogether, just the links from the main distribution's docs.

Closes #20096
Marked find and rfind as stable
auto merge of #20058 : Kimundi/rust/str_pattern_pre, r=alexcrichton

This stabilizes most methods on `&str` working with patterns in a way that is forwards-compatible with a generic string pattern matching API:
- Methods that are using the primary name for their operation are marked as `#[stable]`, as they can be upgraded to a full `Pattern` API later without existing code breaking. Example: `contains(&str)`
- Methods that are using a more specific name in order to not clash with the primary one are marked as `#[unstable]`, as they will likely be removed once their functionality is merged into the primary one. Example: `contains_char<C: CharEq>(C)`
- The method docs got changed to consistently refer to the pattern types as a pattern.
- Methods whose names do not match in the context of the more generic API got renamed. Example: `trim_chars -> trim_matches` 

Additionally, all methods returning iterators got changed to return unique new types with changed names in accordance with the new naming guidelines.

See also https://github.com/rust-lang/rfcs/pull/528

Due to some deprecations and type changes, this is a 

[breaking-change]
auto merge of #20101 : alexcrichton/rust/issue-20096, r=aturon

These crates are all deprecated for their rust-lang/$crate equivalents and by
generating docs we're generating broken links. The documentation for these
crates are generated out-of-tree and are managed separately, so we're not losing
the documentation altogether, just the links from the main distribution's docs.

Closes #20096
Add hypothetical support for ranges with only an upper bound

Note that this doesn't add the surface syntax.
Remove ExprSlice by hacking the compiler

[breaking-change]

The `mut` in slices is now redundant. Mutability is 'inferred' from position. This means that if mutability is only obvious from the type, you will need to use explicit calls to the slicing methods.
Fallout from mut slices
Rebasing and reviewer changes