Clean up the implementations of Bitv and BitvSet.

Functions that add bits now ensure that any unused bits are set to 0.
`into_bitv` sanitizes the nbits of the Bitv/BitvSet it returns by setting the nbits to the current capacity.
Fix a bug with `union_with` and `symmetric_difference` with due to not updating nbits properly
Add test cases to the _with functions
Remove `get_mut_ref`

This is a [breaking-change]. The things you will need to fix are:

1. BitvSet's `unwrap()` has been renamed to `into_bitv`
2. BitvSet's `get_mut_ref()` has been removed. Use `into_bitv()` and `from_bitv()` instead.
auto merge of #17558 : kaseyc/rust/fix_bitvset_union, r=aturon

Updates the other_op function shared by the union/intersect/difference/symmetric_difference -with functions to fix an issue where certain elements would not be present in the result. To fix this, when other op is called, we resize self's nbits to account for any new elements that may be added to the set.

Example:
```rust
	let mut a = BitvSet::new();
	let mut b = BitvSet::new();
	a.insert(0);
	b.insert(5);
	a.union_with(&b);
	println!("{}", a); //Prints "{0}" instead of "{0, 5}"
```