fixed a bug in parsing of parenthesized type expressions

expressions such as `('a * b) * 'c` were totally broken
fixed a couple of tests
added couple of tests for parsing function app over infix arith
added question about Type.Paren to the todo file

It would be nice to get rid of this, but I'm not sure it's possible in
the parsing framework I've settled on (i.e. Pratt parsing for type
expressions)
updated todo