Fix issue 12771 - Variant opIndex for static arrays is not implemented.
Shorten array checks to isArray
Supplemental fix for issue 12857

The local template function zeroLen is inferred to @system. So it should not be placed directly under the @safe unittest body.
Merge pull request #2229 from 9rnsr/fix12857

Supplemental fix for issue 12857
add byCodeunit, byChar, byWchar and byDchar to std.utf
Merge pull request #2188 from Kapps/fix12771

Fix issue 12771 - Variant opIndex for static arrays is not implemented.
Fix #12806

Documentation improvement for `std.traits.isArray` to clarify whether
associative arrays are handled.

Fixes #12806
Reuse codepoint sets for identical character classes

These are very common and blowup memory usage at CTFE (that has no TLS
cache for that).
In any case it makes for slightly more compact memory footprint,
so a net gain regardless.
Merge pull request #2043 from WalterBright/byChar

add byCodeunit, byChar, byWchar and byDchar to std.utf
Merge pull request #2234 from DmitryOlshansky/regex-reuse-sets

std.regex - reuse codepoint sets for identical character classes
Merge pull request #2231 from economicmodeling/fix_12806

Fix #12806