Multiple let bindings vs nested lets semantic differences.

[mattbierner]

the syntax

let x = 1, y = 3 in x + y;

should be functionally equivalent to

let x = 1 in let y = 3 in x + y;

The case:

let x = 3, x = 1 in x;

is currently invalid because x is bound twice. But

let x = 3 in let x = 1 in x;

is valid.

This is related to #60.

[mattbierner]

related to #62