Cache option case constructors

[kerams]

These were constructed from scratch for every single nullable value, resulting in unnecessary garbage and slowdown. So when executing a query like SELECT * FROM generate_series(0, 10000), these 3 reflection-heavy calls would execute 10000 times:

let optionType = typeof<unit option>.GetGenericTypeDefinition().MakeGenericType([| typeParam |])
let cases = FSharpType.GetUnionCases(optionType)
FSharpValue.MakeUnion(relevantCase, args)

[kerams]

Still fairly slow but a 12x improvement over the original.

[<MemoryDiagnoser>]
type Test () =
    let someCtor =
        let t = typeof<unit option>.GetGenericTypeDefinition().MakeGenericType([| typeof<int64> |])
        let cases = FSharp.Reflection.FSharpType.GetUnionCases t
        FSharp.Reflection.FSharpValue.PreComputeUnionConstructor (cases.[1])

    [<Benchmark(Baseline = true)>]
    member _.Reflection () =
        for i in 1 .. 10000 do
            let t = typedefof<unit option>.GetGenericTypeDefinition().MakeGenericType typeof<int64>
            let cs = FSharp.Reflection.FSharpType.GetUnionCases t
            FSharp.Reflection.FSharpValue.MakeUnion(cs.[1], [| box 1L |]) |> ignore

    [<Benchmark>]
    member _.Cached () =
        for i in 1 .. 10000 do
            someCtor [| box 1L |] |> ignore

Method	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
Reflection	54.405 ms	0.7385 ms	0.6908 ms	1.00	1500.0000	-	-	12.44 MB
Cached	4.262 ms	0.0623 ms	0.0583 ms	0.08	132.8125	-	-	1.07 MB

[kerams]

So moving to .NET 5 and most importantly FSharp.Core 5.0 provides a further almost 40x improvement over the Cached result above, so caching the constructor can provide north of 400x speedup.

Method	Runtime	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
Reflection	.NET Core 5.0	47,042.3 us	178.31 us	158.07 us	1.000	1545.4545	-	-	12735.21 KB
Cached	.NET Core 5.0	114.6 us	1.97 us	3.94 us	0.003	95.4590	-	-	781.25 KB

@melanore, @daz10000, can you merge this when you have a spare moment?