[ORC-RT] Add IntervalMap and IntervalSet collections.

IntervalMap is an optionally-coalescing map -- it uses half-open ranges as keys,
allows lookups based on elements of the ranges (returning an iterator to the
containing range) and optionally coalesces adjacent ranges that have the same
value.

IntervalSet is an optionally-coalescing set based on IntervalMap.

These collections will be used to store and lookup metadata section ranges,
e.g. unwind-info ranges.