Implement package-local nicknames

This is a rough first draft of support for package-local nicknames. It does not include any tests, and I am fairly sure the use of mutexes is mostly wrong.

This implementation does pass all of the tests defined by trivial-package-local-nicknames, and does not break any of the existing clisp tests.

I'd be grateful for any review, especially by someone more familiar with the codebase.
Since the HACKING file still points to SourceForge I'm not entirely sure what the best place to send patches is, but I'll start here.

Fixes #20

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