TLAPM parser accepts incorrect use of labels that break operator precedence

[ahelwer]

It is possible to use the label syntax to break operator precedence, and the parser should reject this:

---- MODULE Test ----
op == a * lab :: b + c
====

Quoting from TLA Version 2: A Preliminary Guide [pdf], page 9:

However, the expression is illegal if removing the label would change the way the expression is parsed. For example,
a + lab :: b * c
is legal because it is parsed as a + (lab :: (b * c)), which is how it would be
parsed if the label lab were not there. However,
a * lab :: b + c
is illegal because it would be parsed as a * (lab :: (b + c)) and removing the label causes the expression to be parsed as (a * b) + c.

Ref #159