Fix a crash in Checker Framework issue

[LoiNguyenCS]

No description provided.

[LoiNguyenCS]

Professor, this is the PR to fix #63. Here is what I have done:

1. Add codes to handle the "var" keyword.
2. Fix the codes for solving unsolved union types.
3. Add codes so that Specimin will not create a synthetic class for a file that already exists.
4. Add codes to add the Exception instance in the catch clause to the symbol table.

I have created the corresponding test cases for all of them.

Please take a look. Thank you.

[LoiNguyenCS]

I've just updated the codes a bit to throw an exception when there is a wildcard import.

[LoiNguyenCS]

Professor, I have updated the codes. I have also add two more modifications:

1. A separate method to remove the return type in the declaration of a method, since return type can contain space.
2. In PrunerVisitor, remove every unsolved symbol, since those symbols are not used by target methods.

[kelloggm]

LGTM other than the one minor comment. I don't need to review again.