Implement tuple in the stdlib

[antocuni]

Introduce stdlib/_tuple.spy, which implements tuple in SPy and thus it's automatically supported by the C backend.

Tuples are implemented as structs like this:

struct tuple_int_int {
    int _item0;
    int _item1;
};

Moreover, the "tuple packing" syntax now automatically infers the right tuple type and creates an instance of it: STATIC_TYPE((1, 'hello')) == tuple[int, str].

Similarly, we support tuple unpacking: a, b = sometuple.

The old "blue only, untyped tuple" now lives in __spy__.interp_tuple. It's still used for *varargs.

Unrelated misc changes:

• build libspy.wasm in a way that always include/exports all the symbols which might be needed in tests