The 'codegen_next' crate will eventually be renamed 'codegen'. It
contains procedural macros written with the upcoming 'proc_macro' APIs,
which will eventually be stabilized. All compiler extensions in the
present 'codegen' crate will be rewritten as procedural macros and moved
to the 'codegen_next' crate.
At present, macros from 'codegen_next' are exported from the core
`rocket` crate automatically. In the future, we may wish to feature-gate
this export to allow using Rocket's core without codegen.
Resolves#16.
Sergio Benitez