Imprecise basis merging for nonfix declarations

[shwestrick]

test/fail/basis-merging-nonfix.mlb should fail, but currently it succeeds without error.

The problem: nonfix declarations should override previous infix declarations when merging bases. Currently, they do not.

Suggested solution: while parsing SML source, keep two separate InfixDicts around: one for infix declarations, and one for nonfix declarations. They can cancel each other out when there is any overlap. This allows us to cancel infix/nonfix across source files, as needed for the above test.

[shwestrick]

Thinking about this some more:

• I don't think this example should fail to parse. It will fail to type-check, though (similar to as discussed in Issue with persistence of infixity status #6)
• The suggested solution is messy. Much better is to map each identifier to one of Infix or Nonfix, depending on how it's been set.
• Is there a good example to demonstrate this? Or does it really not matter?

[shwestrick]

Update: I got rid of that test case. Still trying to think of a good example where this issue actually causes a problem.