only block keys with bindings in global-map

Some minor-modes "reclaim" key sequences that are better used through
their "guru" mappings. An example is smartparens, which binds &lt;C-right&gt;
to `sp-slurp-forward-sexp` or drag-stuff, which binds &lt;M-up> to
`drag-stuff-up`. In these instances, we want to check if the key has
been remapped elsewhere before blocking it.

If the binding is the same as in the global map, then guru mode inserts
itself as before, but if it has been changed, then it allows the current
binding to proceed as normal.

This is my first foray into elisp, so any feedback on style would be much appreciated.