Relax constraint in hash_map::EntryRef insertion methods K: From<&Q> to &Q: Into<K>

[tomkris]

Currently EntryRef::or_insert* methods have constraint K: From<&Q> which is required to construct "owned" key from "borrowed" key during insertion operation.

Rust documentation recommends not to use From as trait constraint, and instead prefer to use reversed Into trait constraint:

https://doc.rust-lang.org/std/convert/trait.From.html

Prefer using Into over using From when specifying trait bounds on a generic function. This way, types that directly implement Into can be used as arguments as well.

Changing constraint K: From<&Q> to &Q: Into<K> extends support of insert operation to additional cases where K does not implement trait From<&Q>, but &Q does implement trait Into<K>.

API compatibility: &Q: Into<K> is a strict superset of K: From<&Q> (because of blanket implementation https://doc.rust-lang.org/std/convert/trait.Into.html#impl-Into%3CU%3E-for-T), so this change does not break existing hashbrown API compatibility; all existing code will work with new trait constraints.