Refactor BoundBody into BoundRequestBody

Moves all the logic realted to getting the request body associated
with the request into a separate mod called client::request_bodies. At
least all the tests are there and the code is more organized.

However, this refactor is important because of two reasons:

* First, it prepares BoundRequestBody to receive files. The functions
  now receive the whole EndpointData. In order to implement file
  attachment this will come handy.

* Second, as part of the deprecation of CarteroError, it introduces a
  new kind of error called RequestPreconditionError. Currently it is a
  subtype of CarteroError to make the existing code compatible, but
  because it's a separate enum, it will be easy to unlink once that
  code is touched too.