Improve throughput of {U}Int32/64.{Try}Parse for hex (dotnet/coreclr#18952)

Improve throughput of {U}Int32/64.{Try}Parse for hex (dotnet/coreclr#18952)

* Improve throughput of {U}Int32/64.{Try}Parse for hex

- Copies/modifies the recently added TryParse32/64IntegerStyle routines for hex in order to provide more optimized parsing.
- Since AllowHexSpecifier can't be used with any other styles (other than allowing leading/trailing whitespace), deletes existing code paths for parsing hex.
- As I was copying code around, I kept getting bitten by different names used for the same thing, so I also standardized some parameter names in various helper functions.

* Clean up

- Remove a bunch of unnecessary 'unsafe' keywords
- Rephrase some while(true) loops as do/while loops for consistency and to avoid some repetition
- Remove an unnecessary wrapper function

Commit migrated from https://github.com/dotnet/coreclr/commit/624f72d55a92e49aef3c3cd6e69150fa3b085fac