Maintain hygiene when combining let and ASI

#207

To elaborate: In order to use an infix macro, like `a => a + 1` where `=>` is the macro, we have to be greedy about enforesting expressions in `get_expression`. The first `a` is an expression itself, but what the user intuitively wants is for the whole thing to be a single expression. So we have to try enforesting the next token afterwards, and see if it extends the original expression. With semicolons, it just gets a punctuator back and stops. Without the semicolon, it hits another `const` which kicks off a nested call to `enforestVarStatement` which kicks off let renaming. It then gets back to the original `get_expression` and it decides it has not extended the original expression. So in my example in #207, the `bar` const is actually enforested twice. Once to see if it extends `1` as an expression, and again afterwards.

The problem results from `get_expression`. When we get to the end, we need to return the `peek` which contains the correctly renamed `rest`, instead of just returning `next`. It's very subtle. I also put a guard on the loop to only keep enforesting if the next token is a macro. All we are looking for is to see if the next token is a macro that extends the expression, so it's guarding what we are looking for anyway.