Infer specs from associative destructurings

[vemv]

Given a (speced/defn [{::foo/keys [bar}]), we could observe that the ::foo/bar spec may exist, and add it to the precondition accordingly, against the bar variable.

Note that destructuring may be arbitrarily nested, which calls for a careful implementation.

• Remember that destructuring has two forms (vector + associative) and that associative destructuring may be applied upon arbitrary values (e.g. (let [{valid-msgs true invalid-msgs false} (group-by some? msgs)] ..)