Fixes#93.
Previously, "1 -1" was lexed as two number tokens: a positive literal and
a negative literal. This caused problems when it came to parsing.
Now the '-' and the second number are separate tokens.
Note this is a breaking change, since `-16.sqrt` is now parsed as `-(16.sqrt)`,
as opposed to `(-16).sqrt`.
There is a small bit of overhead to doing it this way, but it might be possible
to optimize that out in the compiler at some point in the future.
Evan Shaw