Extend to other types: NonZero*, tuples, arrays, ...?

[dhardy]

Currently this library is limited to basic numeric types (f*, i*, u*). Should it remain that way?

1. std::num::NonZeroU32 etc. are essentially numeric types. So should we support those?
2. What about numeric and vector types for third-party libraries, behind feature flags?
3. With a few lines of code we can support tuples ((A, B) → (C, D) where A → C, B → D, also for more elements to some limit) and arrays ([A; N] → [B; N] where A → B). It's easy to add, but may impact compile times.

Concern: casting as an operation is expected to be transitive: if a → b and b → c then a → c. Supporting additional "numeric" types such as NonZero* is therefore a substantial increase in the number of conversions we must support.
(We cannot have this automatically since the type of b must be specified.)

Concern: properly casting between types from multiple third-party libraries could quickly become over-complex.

Concern: there is significant appeal in a small, minimal-dependency library.

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Given the above, easy-cast will likely remain limited to standard types. Supporting arrays and small tuples may be desirable.

[dhardy]

Decision: do not support NonZero* types (at least for now). It's a lot of additional casts and I don't have a use-case (although I don't have a problem with someone else making a PR covering NonZero* types similar to how From does, provided it's well tested).

Decision: do try to support third-parties implementing this crate's traits. For now, don't add any such support from here.

Decision: do implement some conversions on array/tuple types.

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Pending one PR, I'll close this.