Fix issue# 10717.

This makes it so that if you're operating on chars or wchars with
std.ascii functions, you don't have to cast the return values back to
char or wchar, but it still allows you to operate on dchars without
mangling any Unicode values.
Add document for memory safety of std.conv.toImpl for char* to string conversion
Merge pull request #1446 from tom-tan/add-conv-toImpl-comment

Add document for memory safety of toImpl for char* to string conversion.

Merged.
Merge pull request #1436 from jmdavis/10717

Fix issue# 10717.

Merged.