Implement the Count property, the IEquatable and IFormattable interfaces, and the...

Implement the Count property, the IEquatable and IFormattable interfaces, and the standard object overrides on Vector64/128/256<T> (dotnet/coreclr#21259)

* Exposing the Count property on Vector64/128/256<T>

* Implementing IEquatable, IFormattable, and the object overrides on Vector64/128/256<T>

* Use StringBuilderCache, HashCode, and the provided format specifier

* Use StringBuilderCache.GetStringAndRelease and get the correct element when formatting Vector64/128/256<T>

Commit migrated from https://github.com/dotnet/coreclr/commit/211d963a42c8988770afa4d2edcbe9be0ed0b8a8