Emit Fable erased unions as TS union types

[alfonsogarciacaro]

This PR stops compiling erased types as Any so they can be dealt with in the Fable2XXX step. @ncave @dbrattli Not sure how it affects Rust and Python, although tests are passing. For JS/TS I'm skipping the declaration and for TS I'm transforming the annotations to TS unions/tuples when possible:

Fable/src/Fable.Transforms/Fable2Babel.fs

Lines 617 to 638 in 6987bc3

	| _, Patterns.Try (Util.tryFindAnyAttribute [Atts.erase; Atts.tsTaggedUnion]) (erasedAtt: Fable.Attribute) ->
	if erasedAtt.Entity.FullName = Atts.erase && ent.IsFSharpUnion then
	let genArgs =
	List.zip ent.GenericParameters genArgs
	|> List.map (fun (p, a) -> p.Name, a)
	|> Map
	let transformSingleFieldType (uci: Fable.UnionCase) =
	List.tryHead uci.UnionCaseFields
	|> Option.map (fun fi -> fi.FieldType |> resolveInlineType genArgs |> makeTypeAnnotation com ctx)
	|> Option.defaultValue VoidTypeAnnotation
	match ent.UnionCases with
	| [uci] when List.isMultiple uci.UnionCaseFields ->
	uci.UnionCaseFields
	|> List.mapToArray (fun fi -> fi.FieldType |> resolveInlineType genArgs |> makeTypeAnnotation com ctx)
	|> TupleTypeAnnotation
	| [uci] -> transformSingleFieldType uci
	| ucis -> ucis |> List.mapToArray transformSingleFieldType |> UnionTypeAnnotation
	// TODO: tsTaggedUnion
	else AnyTypeAnnotation

@Booksbaum An unfortunate consequence is this breaks some of the integration tests for anonymous records casted to interfaces because U2, U3... are not compiled as Any anymore. For now I just commented out the test because I think it's not a common pattern to have an anonymous record with a field like U2<string, int> (you either have string or int) but we can try to bring them back if needed. I also moved the code to deal with this from FSharp2Fable.Util to Replacements.Util, as it's only use from JS and Python replacements.

Also, at the end I added a commit to optimize compile-time equality to reduce generated code for patterns like this (see #3394).