Make char methods const

Make char methods const

`escape_unicode`, `escape_default`, `len_utf8`, `len_utf16`, to_ascii_lowercase`, `eq_ignore_ascii_case`

`u8` methods `to_ascii_lowercase`, `to_ascii_uppercase` also must be made const

u8 methods made const

Update methods.rs

Update mod.rs

Update methods.rs

Fix `since` in rustc_const_stable to next stable

Fix `since` in rustc_const_stable to next stable

Update methods.rs

Update mod.rs