As per RFC 2295, add a Haystack trait describing something that can be
searched in and make core::str::Pattern (and related types) generic on
that trait.  This will allow Pattern to be used for types other than
str (most notably OsStr).

The change mostly follows the RFC though there are some differences in
Haystack trait.

Most notably, instead of separate StartCursor and EndCursors types,
there’s one Cursor type instead.  This eliminate the need for methods
converting between the two types of cursors.

Conversion from cursor to offset isn’t included either since as far as
I can tell it’s not needed.  A generic code operating on Haystack
doesn’t need a concept of offset.  It can operate on cursors instead.

Lastly, rather than range_to_self as suggested in RFC this commit
implements split_at_cursor_unchecked which simplifies default
implementation of strip_prefix_of and strip_suffix_of.

For now leave Pattern, Haystack et al in core::str::pattern.  Since
they are no longer str-specific, I’ll move them to core::pattern in
future commit.  This one leaves them in place to make the diff
smaller.

Issue: #49802

Pattern is no longer str-specific, so move it from core::str::pattern
module to a new core::pattern module.  This introduces no changes in
behaviour or implementation.  Just moves stuff around and adjusts
documentation.

Issue: #49802

Negative delta FTW.

Issue: #49802

Implement Haystack<&[T]> and corresponding Pattern<&[T]> for &[T].
That is, provide implementation for searching for subslices in slices.
This replaces SlicePattern type.

To make use of this new implementations, provide a few new methods on
[T] type modelling them after types on str.  Specifically, introduce
{starts,ends}_with_pattern, find, rfind, [T]::{split,rsplit}_once and
trim{,_start,_end}_matches.

Note that due to existing starts_with and ends_with methods, the
_pattern suffix had to be used.  This is unfortunate but the type
of starts_with’s argument cannot be changed without affecting type
inference and thus breaking the API.

This change doesn’t implement functions returning iterators such as
split_pattern or matches which in str type are built on top of the
Pattern API.

Issue: #49802
Issue: #56345