I propose we make the necessary language changes to support Span

See dotnet/fsharp#4166 (comment)

Interop:

C# 7.2: Ref extension methods on structs
dotnet/csharplang#186
dotnet/roslyn#165

Safety:

C# 7.2: The "readonly references" feature is actually a group of features that leverage the efficiency of passing variables by reference, but without exposing the data to modifications.
https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/readonly-ref.md

C# 7.2: Compile time enforcement of safety for ref-like types. The main reason for the additional safety rules when dealing with types like Span and ReadOnlySpan is that such types must be confined to the execution stack.
https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/span-safety.md

Performance:

C# 7.2:
https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/conditional-ref.md

Proposal: Struct Lambdas
dotnet/csharplang#1060

Proposal: Static Delegates
https://github.com/dotnet/csharplang/blob/master/proposals/static-delegates.md

Proposal: Blittable Types
https://github.com/dotnet/csharplang/blob/master/proposals/blittable.md