linter reference implementation

[bmatsuo-at-luthersystems]

The elps command should implement a linter that can be used directly or used as a reference for implementing linters specific to the application domain elps is being embedded into.

There is one proposed use case for a linter, with docstrings in #46. Other examples would be guarding against direct use of set or using set! when there is no matching variable in scope (though the latter may be better implemented in the long term as a 'compile time' check -- which doesn't exist currently as an elps concept). Another valid concern is attempts to use the in-package operator when not in the package's top-level.

Certainly there are other gotchas which could be detected without too much effort. But the ones discussed here probably merit including a default linter implementation with the project.