Fix #1316: Interpolate interpolations safely
Fix #1316: Interpolate interpolations safely

Instead of compiling to `"" + + (+"-");`, `"#{+}-"'` now gives an appropriate
error message:

    [stdin]:1:5: error: unexpected end of interpolation
    "#{+}-"
        ^

This is done by _always_ (instead of just sometimes) wrapping the interpolations
in parentheses in the lexer. Unnecessary parentheses won't be output anyway.

I got tired of updating the tests in test/location.coffee (which I had enough of
in #3770), which relies on implementation details (the exact amount of tokens
generated for a given string of code) to do their testing, so I refactored them
to be less fragile.

sterpe/coffee-fmt