Introduce UTF Matchers into std.uni.

It's a step zero to get decode-less std.regex.
UTF matchers are efficient functors around a set of
specific tries. Enables processing Unicode characters
without decoding at speeds on par with decoding itself.

Along the way make staticIota at 'package' protected and reuse it.

Fix a shameful typo in setSearcher.
workaround stable sort (std.move) not CTFE-able
const/pure annotations
purify std.uni constructs, blocked by std.algortihm.sort
@safety bags for utfMatchers

Granularity is horribly high. Auto-inference for templates has the
downside that it, leaves no explanations or reasons for failure.
fold in review comments
hide subMatcher until its API and utility is proven
test both narrow strings and range of char for matcher trait
more checking of UTF errors

Overlong sequences, wrong continuation  for UTF-8.
Lone high surrogate for UTf-16/.
address review issues

Spelling, style etc.
another DDoc tweak
Merge pull request #1685 from blackwhale/utf8-matcher

Introduce UTF matchers into std.uni.