Fix lexing of arithmetic expressions without spaces

[aaronbembenek]

Fixes #80

On this program:

rel a(i32)
rel b(i32)
rel ok

a(1-1).
b(2+2).

ok :- a(0), b(4).

we now get:

Parsing...
Finished parsing (0.058s)
Type checking...
Finished type checking (0.004s)
Rewriting and validating...
Finished rewriting and validating (0.13s)
Evaluating...
Finished evaluating (0.002s)

==================== SELECTED IDB RELATIONS ====================

---------- a (1) ----------
a(0)

---------- b (1) ----------
b(4)

---------- ok (1) ----------
ok