* Add retain_mut

This adds a `retain_mut` method to `ArrayVec` and
`TinyVec` that's just `retain` but with mutable
references. This method is present on
`std::vec::Vec` as of a while ago so it's nice to
have in `tinyvec` too.

Fixes #195

* `TinyVec::retain_mut` requires rustc_1_61 feature

It calls `std::vec::Vec::retain_mut` which is only
available since Rust 1.61. People that still want
to use a very old compiler (CI seems to use 1.47)
still get to compile the crate, they just don't
get `TinyVec::retain_mut`.

Note that `ArrayVec::retain_mut` is implemented
inside the crate so it's still available, just the
`TinyVec` format is not.

* Add some tests for ArrayVec::retain_mut

Originally I was going to call this `as_array()`
but `as_inner()` is more consistent with the
existing `into_inner()` method. I don't
_ultimately_ care what it ends up being called.

I wonder if we should have a `impl
From<ArrayVec<T>> for A` implementation or is that
a bit too foot-gun-y?

This lives in its own `impl<A> ArrayVec<A>` block
because rust 1.47.0 is unhappy about having a
const function inside `impl<A: Array>
ArrayVec<A>`. We don't need the trait anyway.

Fixes #193.