implement inherent str constructors

[m4rch3n1ng]

implement #131114

this implements

• str::from_utf8
• str::from_utf8_mut
• str::from_utf8_unchecked
• str::from_utf8_unchecked_mut

i left std::str::from_raw_parts and std::str::from_raw_parts_mut out of this as those are unstable and were not mentioned by the tracking issue or the original pull request, but i can add those here as well.

i was also unsure of what to do with the rustc_const_(un)stable attributes: i removed the #[rustc_const_stable] attribute from str::from_utf8, str::from_utf8_unchecked and str::from_utf8_unchecked_mut, and left the#[rust_const_unstable] in str::from_utf8_mut (btw why is that one not const stable yet with #57349 merged?).

is there a way to redirect users to the stable std::str::from_utf8 instead of only saying "hey this is unstable"?

for now i just removed the check for str::from_utf8 in the test in tests/ui/suggestions/suggest-std-when-using-type.rs.