Rollup merge of rust-lang#39659 - zackw:asciiext-ctype, r=alexcrichton

Add equivalents of C's <ctype.h> functions to AsciiExt.

 * `is_ascii_alphabetic`
 * `is_ascii_uppercase`
 * `is_ascii_lowercase`
 * `is_ascii_alphanumeric`
 * `is_ascii_digit`
 * `is_ascii_hexdigit`
 * `is_ascii_punctuation`
 * `is_ascii_graphic`
 * `is_ascii_whitespace`
 * `is_ascii_control`

This addresses issue rust-lang#39658.

Lightly tested on x86-64-linux.  tidy complains about the URLs in the documentation making lines too long, I don't know what to do about that.