Rename the 'params' argument used in hooks and middleware methods

The name is ambiguous and perhaps even a little misleading. Let's see if we can come up with a more specific name. The argument is a dict-like object containing field names and their values, as defined by a route's URI template.

We should be able to do this without breaking backwards-compatibility.

Start by locating the hooks and middleware methods that receive the dict-like argument, then trace how those values come from a route's URI template. Confirm how the current name is exposed and how backward compatibility should work; done means the argument has a more specific name without breaking existing callers.

Written by the indexing model from the issue text.