Add fn ColList::last(&self) -> ColId

[Centril]

Description of Changes

Adds ColList::last which is faster than list.iter().last().unwrap() thanks to

1. not unwrapping
2. using count leading zeros rather than doing count trailing zeros potentially 63 times.

Will be used in #1240.

API and ABI breaking changes

None

Expected complexity level and risk

2, some unsafe, but well contained.

Testing

• proptests were amended to exercise .last().
• miri was run on the amended test.