Implement rangesets.

The data structure implements non-intersecting intervals over the [0,
UINT64_MAX] range, and supports fast insert, predicated clearing of
subrange, and lookup of an interval containing the specified address.
Internally it is a pctrie over the interval start addresses.

Implementation provides additional guarantees over the structure state
in case of memory allocation failures.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D18893