Move ascii::escape_default to core for no_std support

[ameets]

It sounds like AsciiExt's methods have been largely moved to individual types, which is great. Would it be possible to move ascii::escape_default (escaped version of u8) to core?

Reference: https://internals.rust-lang.org/t/std-ascii-in-core/6263/7

[Phlosioneer]

Triage: #48735 seems to have addressed this?

[SimonSapin]

Yes it has.