Type-check fields + method + function type signatures

[mukul-rathi]

Ensure types assigned to fields + types using in type signatures are valid. Amazing how you can forget to type-check such simple things.

e.g. flag the following uses of NonExistentClass:

class Foo{
    capability Bar;
    var NonExistentClass field : Bar;
    void testSomething(NonExistentClass x) : Bar{
    }
}

function void test(NonExistentClass c){
}