Starting arithmetic parser. At this point, fairly simple: able to capture
variables or numbers, and properly attaches basic operators. Parens are
lexed, but not parsed properly yet (actually, parens not even allowed in
grammar just yet, so technically parens break it)
Fixed up parens. Currently: requiring operators before/after parens. That
is, (1+2x)(2-3x) must instead be written (1+2x)*(2-3x). This is an
unfortunate, but really fairly minor concession, so it will likely persist
unless I figure out an unambiguous way to accomplish the former.