Merge pull request #58 from velour/parser_tests

Allow a trailing , before ) for calls and conversions.
Change SetType to WithType.

The latter is safer, because it returns a copy of the node, leaving
the original safely unchanged.

Fixes #59.
Add ConstConvert.

It's untested.
Add a test that 'const a string = 3' is an error.

It is not a conversion as in string(int(3)).