fe, tests: Type inference for formal arguments

[fridis]

NOTE: This is experimental, would like to gain experience and know if this is useful or does harm.

With this patch, formal arguments types in a feature declaration can be omitted provided that the currently compiled module has a call to the formal argument's outer feature that provides an actual argument that permits type inference.

As an example, here is the first example from https://flang.dev/tutorial/feature_decl_fields

    Point(x, y i32) is

    show (Point 3 4 )
    show (Point 5 12)

    show(p Point) => say "{p.x}, {p.y}"

Type inference permits omitting all formal argument types in this example, so we can omit i32 and Point types for x, y and p and write

    Point(x, y) is

    show (Point 3 4 )
    show (Point 5 12)

    show(p) => say "{p.x}, {p.y}"

This looks like magic, but it is in fact just a removal of redundant information, the language is still fully statically typed. One caveat is that the documentation function of types in the source code gets lost, in particular since the call to a feature may be far away from its declaration, even in a separate file, but always in the same module. Documentation tools like fz -docs operate after the front end is done, so they should have full type information.

This, however, also creates new possibilities for errors: First, there might be several calls to the same feature with incompatible actual argument types. Second, there might be no call at all so the argument type is unknown. Both cases result in new errors.

A new negative test typeinference_for_formal_args_negative checks these error cases.