Update dev-dependencies

This adds a builder to run `cargo miri` in order to check for certain
classes of undefined behavior.

One thing to note is that we've had to parameterize a number of tests to
use a smaller cycle count under miri. This is because miri's stacked
borrows checker is superlinear:

rust-lang/miri#1367

This can cause the miri builder to run out of memory. To avoid this, we
reduce the cycle counts for a few tests that are particularly expensive
when we're running inside miri.

Add miri builder

* Add a new `replace_full` method, which behaves like `replace` but also
  returns the index of the item.

* Clarify that `replace` and `replace_all` do not modify the replaced
  item's insertion order in the documentation.

* Add test coverage for `replace` and `replace_full` by copying the
  existing tests for `insert` and `insert_full`.

Add a `replace_full` method on `IndexSet`