Common checks

[marcoscaceres]

This is mostly a nit... but but it might be good to pull out the "common checks" or "pre-checks" done by .get(), .create(), and store():

2. Assert: settings is a secure context.
3. If options.signal is aborted, then return a promise rejected with options.signal’s abort reason.

And maybe bring in #227 into the above...

Also, I'm a bit dubious about the "Assert: settings is a secure context"... I mean, I get it... but given the it's done by the binding layer, is there ever any chance that it wouldn't be in SecureContext? (Wouldn't we have bigger problems if that ever happened?)