Correct std::ascii mod names in use statements

Renames for std::ascii module:
 * StrAsciiExt => AsciiExt
 * OwnedStrAsciiExt => OwnedAsciiExt

This fixes compilation using today's nightly versions:
 * rustc 0.13.0-nightly (6ef8392b3 2014-10-20 22:17:49 +0000)
 * cargo 0.0.1-pre-nightly (481a433 2014-10-20 05:14:53 +0000)