Bugfix in std.conv.parseEscape

parseEscape was not parsing the escapes:
\
0
'
"
?
unittests for parseEscape
Update unittest to take into account...

...Octal escapes and named character entities. These still don't work, but there was a test checking octal would throw, which is wrong.
Add `std.traits.{isNested,hasNested}`.
Fix `std.traits.FieldTypeTuple` for structs.
Document what exactly `std.traits.FieldTypeTuple` do.
Remove some undocumented, deprecated functions which are slated for removal.

These have been deprecated for a while, but there was some balking
(primarily from Andrei IIRC) at actually removing them when they were
slated for removal, so they were instead made undocumented and slated
for removal after yet another 6 months or so. So, now that that time has
passed, here's another attempt to actually remove them.
Fix issue 4909
[whitespace] Fix spaces in `std.math.FloatingPointControl` example and unittest.
Fix `std.math.FloatingPointControl` two issues.

1. Do not call `setControlState` in destructor if not initialized (previously results in enabling all exceptions).
2. Call `initialize` on setting rounding mode (previously not called).
[docs] Improve `std.math.FloatingPointControl` example.
Enable a bunch of tests for std.net.isemail.

These tests were previously commented out because of a bug
the regular expression module. Now when we have a new regular
expression module we can enable these tests.

This also fixed a bug in the internally used "substr" function.
Merge pull request #1197 from andralex/4909

Fix issue 4909 - Two suggestions for std.algorithm.schwartzSort()
tweak in scoped
Fixes Issue 9794 - std.json cannot handle delete character
Use escape sequence for DELETE character.

This is just to make it a bit clearer what's going on.
Merge pull request #1218 from monarchdodra/scoped2

tweak in scoped
Merge pull request #1219 from jacob-carlborg/issue_9794

Fixes Issue 9794 - std.json cannot handle delete character
Merge pull request #948 from denis-sh/add-traits-for-nested-structs

Add `std.traits.{isNested,hasNested}`.
Merge pull request #1208 from denis-sh/std.math.FloatingPointControl-fixes

`std.math.FloatingPointControl` fixes