[rbi] Make Partition's lazy canonicalization const
Canonicalization only relabels regions. It rewrites the region numbers in
elementToRegionMap so each label is the smallest element in its region, and it
never changes which elements share a region. So it does not change what a
partition means; it is a lazy normalization that the read-only queries run before
they look anything up.
Because canonicalize() was non-const, every query that needs it was non-const
too: getRegion, areElementsInSameRegion, range. A caller holding a const
partition therefore could not ask which region an element is in. It had to copy
the whole partition first, including its element-to-region map, just to be
allowed to ask a question that reads nothing but that map.
Make canonicalize() const, with the body in a private canonicalizeImpl() that a
const_cast reaches, and make those three queries const. The next commit uses this
to stop copying a predecessor's exit partition before knowing whether the walk
will even fork into it.
range() now hands out const_iterator. Nothing mutated through it.
