Add `bool MutableSpan<T>::contains(const T &value)` function, which is an
exact copy of the function from the `Span<T>` class. This makes it possible
to call `.contains()` directly on the `MutableSpan`, instead of having to
call `.as_span()` first.
Note that the `contains_ptr()` function was already copied from `Span` to
`MutableSpan` before.
No functional changes to the affected code, just the removal of the now
no longer necessary `.to_span()` call.
Pull Request: https://projects.blender.org/blender/blender/pulls/131149
eaf596c49c