New generic function FIND-PACKAGE

[Bike]

This simplifies the definition of the state-value protocol and makes it possible for clients to use an alternate package system without having to redefine interpret-symbol and the #+ #- macros.

I'm not sure how to provide a generic recovery for not finding a keyword package. The extended package prefix code is also probably a problem as it ought to signal an error, and that error would ideally refer to the stream and the position in it, which find-package is not privy to. Right now if the package doesn't exist the extended package prefix code will bind the *package* aspect to nil, which is probably a type error by default but is definitely not the best.