Consider implementing a lexicographical comparison algorithm like `std::equal`

We have `hana::equal` to compare arbitrary objects and sequences, and we have `hana::lexicographical_compare` which is like `std::lexicographical_compare` (hence for `Iterable`s only). It might be nice to have some generic algorithm that does like `std::equal` (compare for equality of `Iterable`s), which we could then use to implement `hana::equal` for sequences.

Start by reading the implementations and documentation for hana::equal and hana::lexicographical_compare, then compare their intended behavior with std::equal. Define the generic Iterable comparison semantics and determine how hana::equal for sequences would use it. Done means the API behavior is agreed, implemented, and covered by relevant sequence and Iterable tests.

Written by the indexing model from the issue text.