Change AsSpan() on OwnedMemory to property Span and rename AsMemory

[ahsonkhan]

Introduced in dotnet/coreclr#13583.
There is currently a property on OwnedMemory called AsMemory. This is inconsistent with the OwnedMemory.AsSpan() method. Therefore, change the AsSpan() method into an abstract property named Span and rename the AsMemory property to just Memory.

dotnet/coreclr#13583 (comment)
dotnet/corefx#23701 (comment)

cc @KrzysztofCwalina, @stephentoub

[KrzysztofCwalina]

Before we do this change, can we discuss (at the API review) when we intend to use properties and when methods for span/memory related conversions? I

[ahsonkhan]

can we discuss (at the API review) when we intend to use properties and when methods for span/memory related conversions?

cc @terrajobst, @karelz, @shiftylogic