[std::ascii] Add .to_ascii_opt() and .into_ascii_opt() returning Option

… instead of failing.

Make them default methods on the trait, and also make .to_ascii()
a default method while we’re at it.

Conflicts:
	src/libstd/ascii.rs
auto merge of #10828 : SimonSapin/rust/ascii_opt, r=pcwalton

… instead of failing.

Make them default methods on the trait, and also make .to_ascii() a default method while we’re at it.