[Repo Assist] refactor: add save-based caching to GetField/GetEvent/GetNestedType; use Dictionary in ILNestedExportedTypesAndForwarders

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Summary

Two complementary consistency/performance improvements to ProvidedTypeDefinition and ILNestedExportedTypesAndForwarders.

1. save-based caching for GetField, GetEvent, GetNestedType

Before: these three overrides used Array.filter / Array.tryFind for O(n) linear scans on every call:

override this.GetField(name, bindingFlags) =
    let xs = this.GetFields bindingFlags |> Array.filter (fun m -> m.Name = name)
    if xs.Length > 0 then xs.[0] else null

After: they now use the same save-based dictionary cache that GetMethodImpl and GetPropertyImpl already use — building a name → member dictionary once per bindingFlags combination and doing O(1) lookups thereafter:

override this.GetField(name, bindingFlags) =
    let table =
        save (bindingFlags ||| BindingFlags.GetField) (fun () ->
            this.GetFields bindingFlags |> Seq.map (fun f -> f.Name, f) |> dict)
    match table.TryGetValue name with | true, f -> f | false, _ -> null

Discriminator bits chosen to avoid cache-key collisions with existing save calls:

Member	Discriminator bit
GetMethodImpl	BindingFlags.InvokeMethod (0x100) — existing
GetPropertyImpl	BindingFlags.GetProperty (0x1000) — existing
GetField	BindingFlags.GetField (0x400) — new
GetEvent	BindingFlags.SetField (0x800) — new
GetNestedType	BindingFlags.SetProperty (0x2000) — new

The stale //save ("field1", ...) / //save ("event1", ...) / //save ("nested1", ...) comments (from a previous refactor attempt) are also removed to reduce noise.

2. ILNestedExportedTypesAndForwarders: Map → Dictionary

Before: used a functional Map (balanced BST, O(log n) lookup) built via Array.fold:

let lmap = lazy ((Map.empty, larr.Force()) ||> Array.fold (fun m x -> m.Add(x.Name, x)))
member __.TryFindByName nm = lmap.Force().TryFind nm

After: consistent with the adjacent ILTypeDefs and ILExportedTypesAndForwarders types which already use Dictionary:

let lmap = lazy (
    let m = Dictionary()
    for x in larr.Force() do m.[x.Name] <- x
    m)
member __.TryFindByName nm = match lmap.Force().TryGetValue nm with true, v -> Some v | false, _ -> None

Test Status

All 126/126 tests pass (dotnet test tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release).

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