Add Raise and two macros for setting the error msg

Usage:

Raise :some-error "Some error message"
Remove several big memory leaks

When running compiler.vu (the prototype for the new Déjà Vu compiler
written in Déjà Vu!), Valgrind now reports that 20% of the memory in
use is inaccessible, instead of the almost 40% before this commit.

The remaining 20% remains elusive for now.