[Issue 137] Trigger eldoc after symex motions

[pbaille]

Summary of Changes

As dicussed in issue 137 eldoc is no longer working well.

As discussed, the def-symex-motion macro has been introduce in order to keep track of the list of available motions.

Those motions can be fed to eldoc-add-comand (during symex-initialize) in order to restore eldoc behavior.

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[countvajhula]

Looks great, thank you @pbaille !