perl: Rename all mal classes to begin with "Mal::".

perl: Rename all mal classes to begin with "Mal::".

Perl package names (and hence class names) are global, meaning that if
any module that we use loads a module called "Symbol", it will clash
with mal's Symbol class.  It turns out that something does this already,
which breaks inheritance from mal's Symbol class.  Since most of mal's
class names are very generic, it seems wise to rename all of them.