Fix parsing algorithm.

Before that it was impossible to use Ruby interpolation syntax because
these transformation were to take place:

	`#{foo}`  => value of Haskell variable `foo`
	`#\{foo}` => `#\{foo}`

Neither of those is acceptable, because Ruby wants `#{foo}`.
This patch changes transformations to:

	`#{foo}`   => value of Haskell variable `foo`
	`#\{foo}`  => `#{foo}`  -- Yay!
	`#\\{foo}` => `#\{foo}` -- Still possible.