Speed up trie construction by factor of ~3.

Take advantage of word-at-once checking for slices of bit-packed
arrays in the trie.
Merge pull request #1973 from yazd/fix-typos

[Trivial] Fix documentation typos
Merge pull request #1969 from blackwhale/faster-trie-build

std.uni - speed up Trie construction by factor of ~3
[Trivial] Documentation change for std.conv