Fix intermediate variable declarations

[adam-mcdaniel]

In Sage, variables are pushed onto the stack when they're declared. This leads to issues in expressions like this:

let x = 5 * match 'a' {
    'a' => 6,
    'b' => 7,
    'c' => 8
};
println(x);

Here, the match expression generates an intermediate let binding for the bound expression 'a'. It thinks that this variable is stored just above the last variable on the stack, but 5 is actually stored there, the stack frame has grown in the mean time. So, this PR adds tracking of the stack frame for intermediate value and correctly compiles the code above.