Accidental self-references can cause stack overflows

[ankrgyl]

let a = unsafe select a from foo.t;
let b = unsafe select count(*) from a;

a;
b;

I suspect this is related to #14, specifically that we add a to the namespace before we're binding the expression. This makes sense for recursive functions but not for variable declarations.

[ankrgyl]

The error looks like:

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
Abort trap: 6