std: Change CharEq to take `&mut self`

This is similar to the previous commits to allow invocation of a closure through
a `&mut self` pointer because `&self` is disallowed. One of the primary
implementors of the CharEq trait is a closure type, which would not work if the
method continued to have `&self`.

In addition to changing mutability of the `matches` method, this modifies the
following methods from &CharEq to take a type which implements CharEq by value.

* trim_chars
* trim_left_chars
* trim_right_chars

Where these methods were previously invoked via

    s.trim_chars(&'a')

it would now be invoked through

    s.trim_chars('a')

[breaking-change]

0 files changed, 0 insertions, 0 deletions