Add a Span-based constructor overload to PooledStack

[jtmueller]

Is your feature request related to a problem? Please describe.

If foo is PooledList<int>...
new PooledStack<int>(foo.Span)
...does not work without an explicit cast.

Describe the solution you'd like
It would be nice to not have to cast. There may be other pooled collections to which this applies.

[jtmueller]

Bitten again by F#'s lack of support for implicit casts. This is not needed.