Tony Finch authored May 22, 2018 and Daniel Salzman committed Aug 20, 2018

No functional change.

The point of this commit is to make it possible to stash a few
flags in leaf nodes as well as branch nodes, though we don't
yet exercise this possibility in any meaningful way.

This fixes a foolish mistake in the original qp trie data structure
declarations: the combination of unions and bitfields is a complete
disaster for portability, and it gets into dangerous territory wrt
compiler optimization. Instead, we just use a big enough word (uint64_t)
which is broken up into fields using accessor macros and inline functions,
and cast it to a pointer when necessary. We don't actually care about
the detailed layout in memory, just the numeric value, so a union was
the wrong tool.