This PR implements linear time and constant space algorithms for a few of the interval operations:

• canonicalize: track the current "merge target", and merge each interval into the target if they overlap. The merge target is always shifted left as appropriate, so that we can simply truncate the list afterwards.
• negate: replace each interval with the negated interval to its left, then finally add the trailing interval on the right. Take care to avoid the leftmost and rightmost intervals if they're empty.
• union: two possible algorithms:
  • If the current vector has enough extra space: merge the two lists, in-place, into the vec, then perform a canonicalization.
  • If the current vector needs to be reallocated: merge the two lists of ranges with an iterator that outputs them all in sorted order. Merge them as they come, if they're adjacent, and construct a new vec out of them.
• push: use a binary search to insert the interval, the use the in-place sorted union.