Add coercion for explicit parameters of partially applied function

[GuntherRademacher]

As discussed in qt4cg/qt4tests#148, coercion of explicit parameters is part of the process of constructing the function item, that results from partial function application.

This change thus adds coercion to PartFunc.item.

This causes some type errors to be detected earlier, fixing QT4 test cases

• FunctionCall-422
• xqhof40
• xqhof41