a start on std.regex conversion

few steps more
hacks on the way to integrate new uni into std.regex
workaround internal error
adjust Tries back to 2-level
another minor tweak, always use tries not sets for now
hack our way to make ctRegex work, some tests disabled
Kickstart not quite ready, casefolding too. Otherwise looks fine
introduce simpleCaseFolding to support case-insensitive regex
fix range violation for non-existant sets of length < 2
make simpleCaseFoldings CTFE-able
get our case-insensitive regex back
reanble kickstarter engine in std.regex
Get rid of internal.uni, internal.uni_tab

Saving amount to ~290Kb on 32bit.
fix 64bit bitrot
fix issue 11784

Actually add a test case, the issue was fixed as part of the set
of commits that precede this one.
Eanble kickstart engine for Thompson as well
Enable again linear scans for small sets in std.regex
Enable latent optimizations in std.uni

Simplify set construction.
Use direct slicing for CowArray, makes for faster debug builds/CTFE code.

Also use Gallop search policy for CodepointSets, as it's closer to
to the common cases for merging charsets.
Workaround curious CTFE bug