Test and fix CONCATENATE for arbitrary types

[brpocock]

Per string.lisp:

A simple CONCATENATE that works for strings and vectors. It does not
accept all of the target object types that it should, and does not
correctly handle checking if the type of elements in the target can
accept the types of elements in the objects to be concatenated (TODO).

In other words: (concatenate 'string one-string another-string) is fine,
but more complex cases like mixing strings and vectors are probably not
handled correctly yet.