[Optimization] Infer or respect IsReadOnly on all structs to remove unnecessary copies

[NinoFloris]

C# has the possibility to mark types and members as readonly on structs to signal to roslyn no defensive copies have to be performed. F# with its immutable by default semantics should take advantage of this to gain end users some automatic perf improvements.

Relevant language feature docs:
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct#readonly-struct

[NinoFloris]

Update: IsReadOnly on a method is optimized today in the compiler

fsharp/src/fsharp/infos.fs

Lines 1280 to 1281 in 6b1af12

	| ILMeth (g, ilMethInfo, _) -> ilMethInfo.IsReadOnly g
	| FSMeth _ -> false // F# defined methods not supported yet. Must be a language feature.

(thanks @baronfel). However for methods that are part of a readonly struct roslyn does not emit the attribute for every method, and the F# compiler seems to be missing a lookup on the struct itself for readonly to mark all members as such.

[T-Gro]

@NinoFloris you are right, and it is actually even worse.

If a struct is marked as readonly, Roslyn removes the readonly attribute on method level, and only keeps the top level one on the struct itself.
This is definitely helpful, I will have a look and what can be done.