A tuple pattern for built-in signatures #344

A built-in's signature can't say "a tuple of any length", so first, last and zip have one overload per tuple length, up to twelve (#342), as Rust's standard library does for tuples. Longer tuples fail analysis, though the implementations take them.

Proposal: a tuple pattern in signatures, such as <A>((A, ..)) -> A for "a tuple of at least one element, starting with A", or <T..>(Iterable<T>..) -> List<(T..)> for zip. It's a new type form, so every match on tuple types in the analyser, the VM's conformance checks and the LSP has to handle it; worth it once more built-ins need it.

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