Add Request object to hook package to simplify API

[moorereason]

To avoid having to pass around so many parameters to the hook package,
create a Request object to store all request-specific data. Update APIs
accordingly.

[moorereason]

@adnanh,
I'm submitting this PR as a proposal since it will likely create a bunch of merge conflicts for your context branch. Although, if I had done this earlier, you could have simply added a Context to the Request and not had to update so many function signatures. If you want to wait on this until your context work is complete, I'm okay with that.

There's no functional change to the user. This is simply a simplification of the internal hook API.

Let me know what you think.

[adnanh]

Perfect, merging right away :-)