Fixed bug in Hash#invert
Major speed boost to reloading marshalled Hamster::Hash
Pulled out some repeated code in Hash into new method
Added default implementation of #inspect to Hamster::Enumerable
Added default implementation of #pretty_print to Hamster::Enumerable
Pull out repeated code in AVLNode into a new method
Making specs stricter (use eql? rather than ==)
Working on style of specs (using context rather than describe)
Removed Vector#cons (the name is very misleading)

Vector#cons doesn't mean anything to people who have never coded Lisp. To those
who have, the name is misleading. It implies allocating something like a linked
list node which points to the Vector.
Moved specs into spec/lib (to match directory structure of library source)