make Char no longer a subtype of Integer

Char now supports a limited set of "integer like" behavior.

* comparisons with integers
* Char - Char = Int
* Char + Int = Char

update docs and add a NEWS.md entry
Merge pull request #8816 from JuliaLang/jcb/char_not_integer

Make Char no longer a subtype of Integer
Merge branch 'upstream-master'