More efficient implementation of Gen.apply

[TysonMN]

This is a spin off from #260 (comment).

Gen.apply is currently implemented by calling Gen.bind.

fsharp-hedgehog/src/Hedgehog/Gen.fs

Lines 46 to 48 in 6b376a6

	let apply (gf : Gen<'a -> 'b>) (gx : Gen<'a>) : Gen<'b> =
	bind gf <| fun f ->
	bind gx <| (f >> constant)

This works but doesn't have the best performance. In particular, I think the current implementation exacerbates recursion that lacks tail calls.

[TysonMN]

I don't think this is an issue any more for two reasons.

First, I thought implementing apply via bind was possibly the sole cause of why certain code resulted in a stack overflow. It is at not the most fundamental issue since map along suffers from stack overflows (c.f. #289).

Second, I wondered if the applicative behavior of apply and the monadic behavior of bind were different. I now think they are the same.

[TysonMN]

Second, I wondered if the applicative behavior of apply and the monadic behavior of bind were different. I now think they are the same.

I was wrong; these behaviors are different. This blog post does a good job explaining the difference.