Fixing the error Sandy Knight reported: x>= -10 wasn't counted as simplified.

It was being converted to the expression x+-1*-10(=0).
Now it gets converted to x+10(=0);

Collecting here doesn't hinder the isSimplified() check because both halves
of an equation are also considered separately for simplification before
being combined.

Test Plan:
Added test cases to src/expressiontools/test.html

Auditors: alpert