Allow coercing elgible variants to string/int/float

[zth]

This lets you coerce variants to their underlying string/int/float runtime representation, when applicable.

// These are represented by strings at runtime
type x = | One | @as("two") Two

let d = One

// This now works
let f = (d :> string)

type d = | @as(1) One1 | @as(2) Two2

let c = One1

// This also works now because all constructors are represented as ints at runtime.
let dd = (c :> int)

[cristianoc]

Looking great.
Just some comments on code organisation.

[cristianoc]

Great!
Just a tiny observation
Also, changelog