Support mutually recursive local definitions (let rec)

[robotlolita]

Currently let introduces a new scope, so it's not possible to write mutually recursive definitions:

let x = y in
let y = 1 in
x

Will give you a y is not defined error, since y is not in x's scope. The solution for this is to introduce all definitions at the same time (we don't need a rec modifier since all bindings are lazily evaluated anyway):

let x = y
let y = 1
in x