Allow Memory<T> to be target of collection expressions

[petrroll]

While Span is very much a first class citizen and participates in new C# features (collection expressions, utf8 string literals #9493) Memory seems to be missing. And while Span is usually enough for consuming data, it is frequently impossible to use for producing them (async network servers).

As such it is unfortunate that:

ReadonlySpan<byte> = [2, 3, 4]; // works
ReadonlyMemory<byte> = [2, 3, 4] // doesn't work

And similarly on callsites, etc. It's most annoying when trying to pass an empty "memory", where instead of [] one has to use older/more verbose static empty things that can be converted to readonlymemory and/or default which is less readable.

[timcassell]

According to dotnet/runtime#111715 (comment), it will be unblocked by #9009.

Tbh, I think (Readonly)Memory<T> should just implement IEnumerable<T>, so we can foreach over them directly (dotnet/runtime#23950), and this could be implemented without any language changes, but that's a BCL decision.