Rename string-slicing extension methods (dotnet/corefx#27328)

Rename string-slicing extension methods (dotnet/corefx#27328)

* Rename string-slicing extension methods

As part of

https://github.com/dotnet/corefx/issues/26894

the api folks have approved renaming AsROSpan and AsROMemory
on string instances to AsSpan and AsMemory (as the "readonly"
is obvious given the read-only nature of the input.)

This puts the renaming in effect. Basically a big search-replace
commit.

* Fix OpenSSL build break

* I see this is going to be a treadmill

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>