v0.2.3

Rewrite using a tokenizer to support upcoming multi-char keywords and functions.
As a consequence of this rewrite, and this new direction, spaces now need to be put in between 'a' for them to register as implicit multiplication of the previous answer, otherwise they will be read as an invalid keyword.

v0.2.2

Show user where the error in parsing their input occurred, if possible.

v0.2.1

zig calc: rename and add test cases