Implemented line indexing for faster jumps/looping.

This required adding the ability to to get and set the
tokenizer's current position pointer from outside of it with
tokenizer_pos and tokenizer_goto (c.f. tokenizer_init).
The index itself is stored as a linked list, which makes it ripe
for further optimization. It is accessed with index_* routines.
The interpreter resorts to running the old, slower jump procedure
when jumping forward (to a line that hasn't been index).

New goto test.