Fix appender form elaborate assign types
More unittest for appender
tweak appender, add unittests for not-yet covered paths
Remove conditional emplace call
Fix missing `void back(bool)` in Array!bool.Range

Forked from #1010

Also some cleanup:
changed ulong to size_t, some spaces, some Tuples, some "!(T)"=>"!T"
improve Fft comments: fix typo 'nterleaved'; add description of FFT conventions
Merge pull request #1656 from monarchdodra/boolArrayBack

Fix missing `void back(bool)` in Array!bool.Range
Changed to return uninitialized Variant when the stored function returns void.
Add unittests for issue 11360.
Slightly reworded comment.
Fix issue 11361 - Variant fails to compile with delegates containing const arguments.
Merge pull request #1666 from Kapps/fix11360

Fix issue 11360 - Function pointers / delegates returning void fail to compile with Variant.
Resolve merge conflict.
Merge pull request #1667 from Kapps/fix11361

Fix issue 11361 - Variant fails to compile with functions containing const arguments.
Merge pull request #1529 from monarchdodra/appenderEmplace

Fix appender form elaborate assign types
Remove explicit pure
Merge pull request #1655 from monarchdodra/arrayEmplace

Remove conditional emplace call
Merge pull request #1664 from majiang/master

improve Fft comments: add description of FFT conventions