refactor(API): Distinguish between different types of helpers

Divide the helper methods into three groups:

1. Internal functions that are specific to the API class but do not
require access to self. These go in "api_helpers".

2. Functions that are used for the API class but may also be useful
to higher-level or custom web frameworks building on Falcon. There
are presently two such functions, both related to routing, so they
go into a new module, "routing". These functions will be included
in Falcon's reference documentation, and care must be taken to
avoid introducing breaking changes to them in future versions of
the framework.

3. Functions that only need access to self for performance reasons,
which would otherwise reside in "api_helpers". These go inside the
definition of the API class itself.