Var metadata for multimethods created by `defmulti` doesn't get updated when metadata on symbol changes

[camsaul]

For example if you make it ^:private, or move it a few lines, this change isn't picked up until the multimethod is completely redefined.

We should update the metadata for the var, but should not redefine the entire multimethod.

This should apply to metadata passed as part of the macro body as well (docstrings and attributes map) -- don't redefine the multimethod when those change.