Better error messages when students use custom types in collections.

[htiek]

This edit introduces a bunch of extra static_assert checks that give cleaner error messages when students interact with our container types. For example, if you make a Vector<T> for a custom type T that isn't hashable, then calling hashCode on that vector gives a sensible error message about needing to define a hash code for the custom type. The same is true for relational operators, stream insertion, and stream extraction, and these assertions are there for all our containers, not just Vector.

This is essentially a fix for #65 but goes much beyond the Vector type.