Print single instead of double equals sign

[LincB]

ACT currently prints the E_EQ expression with a double equals sign, but the parser expects a single equals sign, so some expressions printed using the ACT tools aren't parsable. This commit makes the expression print function use a single equals sign.

Since the expression printing code is widely used, there may be potential consequences to this change that I don't anticipate, so please let me know if this change isn't possible for some reason.

[rmanohar]

This looks good, but it breaks one of the test cases. The test case "golden output" needs to be updated (it prints out an error message with ==, which now changes to =). Could you submit an updated pull request with this change?

Run "make runtest" in the act/ directory to see the failing case. The failing case is misc/8.act. To update the correct test case output, run ../validate_subdir.sh 8.act in the misc/ directory. That creates the "golden output" for the test case based on the act-test binary. Now make runtest should pass. git status will show you what was modified.

The new pull request should have your expr2.cc change, plus the new misc/runs/8.act.stderr

[LincB]

I added a commit to this pull request with the updated golden output. Please let me know if there are any other issues!