Add comprehensive equality specs

[nex3]

Currently, specs for value equality (like most specs) are spread haphazardly throughout the repo. We should organize them consciously and make sure all edge cases are being tested. Equality specs for each kind of value should live in that value's directory (values/$type/equality.hrx), and they should share assert-equal() and assert-unequal() mixins that verify not only that the values are equal according to == but that all functions that care about equality (index() and various map functions) consider them equal as well.

[stof]

@nex3 I think a comprehensive set of specs also need to include another kind of tests: tests about equality between different types of values (where different types are never equal, except for the empty list and the empty map).