Flatten nested 'let' terms for transformation into Java

[joshsh]

When we map 'let' terms into Java, we turn the individual bindings into local variables, taking care to get the order of variable definitions right, and wrapping them in a container for delayed evaluation in the case of recursive variables.

However, nested 'let' terms are currently unsupported. For example:

let a = 1
    b = let x = 10
            y = 20
            in x + y
    in a + b

This is a perfectly reasonable term in Haskell, but in order to map it into Java, we will need to flatten it first, e.g.

let a = 1
    b = b_x + b_y
    b_x = 10
    b_y = 20
    in a + b

Now we have siblings which reference each other, but this is supported so long as there are no cycles. This flattened structure can be mapped directly into Java.

[wisnesky]

That this transformation is semantics preserving follows from the associativity law for monads! https://wiki.haskell.org/Monad_laws