Update std.uni tables generator to use @nogc

Enables @nogc isWhite and related functions.
A supplement to pull #2323
Merge pull request #2373 from DmitryOlshansky/gen-uni-nogc

Update std.uni tables generator to use @nogc
Make TakeExactly!R implicitly convertible to Take!R
Merge pull request #2370 from JakobOvrum/takexactly_take

Make TakeExactly!R implicitly convertible to Take!R