Add FS-1047 (#67)

Adds `match!` keyword per
[RFC-1047](https://github.com/fsharp/fslang-design/blob/main/FSharp-4.5/FS-1047-match-bang.md).

I'm not quite sure if I've done this one correctly. I didn't exactly
understand the "weird section".

---------

Co-authored-by: Edgar Gonzalez <egonzalez@totallymoney.com>

Author: roboz0r

spec/expressions.md

@@ -135,10 +135,11 @@ comp-expr :=
     use pat = expr in comp-expr
     yield! expr                     -- yield computation
     yield expr                      -- yield result
-return! expr                        -- return computation
+    return! expr                    -- return computation
     return expr                     -- return result
     if expr then comp - expr        -- control flow or imperative action
     if expr then expr else comp-expr
+    match! expr with pat -> comp-expr | ... | pat -> comp-expr
     match expr with pat -> comp-expr | ... | pat -> comp-expr
     try comp - expr with pat -> comp-expr | ... | pat -> comp-expr
     try comp - expr finally expr
@@ -702,6 +703,7 @@ expr { yield! ... }
 expr { try ... }
 expr { return ... }
 expr { return! ... }
+expr { match! ... }
 ```
 
 More specifically, computation expressions have the following form:
@@ -786,6 +788,8 @@ Then, T is defined for each computation expression e:
 
 **T** (match e with pi - > cei, **V** , **C** , q) = **C** (match e with pi - > {| `ce` i |} 0 )
 
+**T** (match! e with pi - > cei, **V** , **C** , q) = **C** (let! p = e in match p with pi - > {| `ce` i |} 0 )
+
 **T** (while e do ce, **V** , **C** , q) = **T** (ce, **V** , v. **C** (b.While(fun () -> e, b.Delay(fun () -> v))), q)
 
 **T** (try ce with pi - > cei, **V** , **C** , q) =

spec/lexical-analysis.md

@@ -309,7 +309,7 @@ The following symbolic or partially symbolic character sequences are treated as
 
 ```fsgrammar
 token symbolic-keyword =
-    let! use! do! yield! return!
+    let! use! do! yield! return! match!
     | -> <-. : ( ) [ ] [< >] [| |] { }
     ' # :?> :? :> .. :: := ;; ; =
     _? ?? (*) <@ @> <@@ @@>

spec/rfc-status.md

@@ -34,7 +34,7 @@
 | F# 4.1 | FS-1027 | [Complete Optional DefaultParameterValue](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1027-complete-optional-defaultparametervalue.md) | |
 | F# 4.1 | FS-1029 | [Implement IReadOnlyCollection in list](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1029-Implement%20IReadOnlyCollection%20in%20list.md) | |
 | F# 4.1 | FS-1040 | [Enum Match](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.1/FS-1040-enum-match.md) | |
-| F# 4.5 | FS-1047 | [Match Bang](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.5/FS-1047-match-bang.md) | |
+| F# 4.5 | FS-1047 | [Match Bang](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.5/FS-1047-match-bang.md) | [completed](https://github.com/fsharp/fslang-spec/pull/67) |
 | F# 4.5 | FS-1053 | [Span](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.5/FS-1053-span.md) | |
 | F# 4.5 | FS-1054 | [Undent List Args](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.5/FS-1054-undent-list-args.md) | |
 | F# 4.5 | FS-1055 | [Subsumption for Yield in Sequence Expression](https://github.com/fsharp/fslang-design/tree/main/FSharp-4.5/FS-1055-subsumption-for-yield-in-sequence-expression.md) | |