Implement nom::sequence::Tuple for () (Unit)

[LoganDark]

Right now, () (Unit) does not implement the nom::sequence::Tuple trait required to use the tuple combinator. This makes it difficult to use in macros.

Example

I'm currently mass-producing parsers using a macro for a fictional instruction set assembler. Each instruction may take a variable number of arguments so I am using this macro component to parse all arguments:

		tuple(($(
			preceded(space1, $parser)),*
		))

This works fine for any number of arguments >=1 and <=21, but notably does not work for a number ==0. Here's why:

the trait bound `(): nom::sequence::Tuple<_, _, _>` is not satisfied

Looks like () does not implement Tuple, even though it could concievably implement a tuple of zero elements.

This pull request implements Tuple for () such that it will always succeed and it is always a no-op. It also adds a test case that verifies that tuple(()) never fails and always returns the input unchanged.

I am unsure if the ParseError<I> bound is needed on the impl, but I figured it's better to be safe than sorry and stay consistent with the existing tuple impls. I would also appreciate assistance with removing the ugly turbofishes from the test.

[coveralls]

Coverage increased (+0.004%) to 79.568% when pulling 476f503 on LoganDark:unit-tuple into 294ffb3 on Geal:main.

[Geal]

good idea, thanks

[LoganDark]

It looks like the way you merged the PR clobbered my GPG signature, so it's showing as unverified in the commit log. Unfortunate :/ although my original commit from this PR is signed, so it's not a huge deal.

[Geal]

Right, I generally use the "squash and merge" to make one commit per PR and link it to the PR in the title