Product types in ALFE

This is part of the ALFE types series.

(Foo, Bar) is a "product" type, aka Tuple<Foo, Bar> or a structure with unnamed fields. Tuple is another variadic template, so (Foo, Bar, Baz) is syntactic sugar for Tuple<Foo, Bar, Baz>. Values of Tuple type are written in the same way as the type itself, except with values instead of types inside the parentheses: (foo, bar). You can have tuples of LValues as well as RValues, which is handy for functions that return Tuple values:

(foo, bar) = getFooBar();

You can also have a tuple of declarations:

(Foo foo, Bar bar) = getFooBar();

(Foo) is identical to Foo just as it is in C, but in ALFE it's also a 1-tuple (so 1-tuplifying is really just a no-op). There is also a single 0-tuple type () aka Tuple<> which is another name for Null. There is a single value inhabiting this type, which is also (by slight abuse of notation that took me a while to determine wasn't too amiguous or confusing) called () and Null (a type specifier can be used as a value if it is default-constructable).

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