Optimize conversion of Key enum from/to string, reduce allocations (d…

…otnet#9697)

* Simplify CanConvertFrom

* Simplify CanConvertTo

* Speed up and simplify ConvertFrom, add fast-path for F-keys

* Speed up and simplify ConvertTo, add IsDefinedKey helper

* Add documentation, formatting adjustments only

* Use IsDefinedKey in CanConvertTo as well

* Remove redundant usings

* Fix an unlikely IndexOutOfRangeException we could have introduced

* Prevent behaviour change, let it throw an exception

* Address PR feedback

* Address nits, fix spelling