[compenguy]

Ensure matching arity of structs at the type level by encoding as a const generic on the struct.

Missing from this change is:

• insurance that the arity isn't zero (blocked on some std::num::NonZero* work to make it work with const generics)
• simplify basic usage a little by providing a default ARITY of 2 (blocked on rust issue #44580 default values for const generic parameters are experimental)

The first change would be compatibility breaking, the second is not (pretty sure, anyway).

[compenguy]

Rejecting this PR because:

1. it complicates the API, requiring specifying generic parameters and lacks the clarity of a named method
2. it appears to have a negative performance impact

I'm willing to reconsider someone is willing to invest some effort in mitigating some of these issues.