Compile sprintf where possible

[charlesroddie]

String formatting functions sprintf, printf, etc. are popular in F# code, but currently slow and aot-unfriendly, implemented using reflection and dynamic codegen.

They should be implemented with consideration for performance, and for compatibility in reflection-constrained environments.

Most commonly sprintf occurs in conjunction with a known format string. Therefore one approach would be to recognize the pattern sprintf specificStringFormat and compile this combination.

Pros: performance and aot-compatbility
Cons: adds code to maintain, as removing the existing approach would be a breaking change.

type R = { Field:int }
let printR:R -> string =
    // Matches `sprintf specificStringFormat` so is compiled
    sprintf "r is %A"
let printTwice<'a>(f:Printf.StringFormat<'a->string>) (x:'a) =
    let s =
        // Not compiled and uses existing approach, since `f` is not known statically.
        sprintf f x
    s + s

[charlesroddie]

Closing temporarily in favour of fsharp/fslang-suggestions#919 which is likely to be more natural to implement in the short term and a stepping stone to sprintf compilation.