cleanup up imports with Moo

 * Any modules 'use'-d before the 'use Moo::Role' statement will have
   their import cleaned automatically.  Any modules 'use'-d before the
   'use Moo' statement will have their imports cleaned by namespace::clean
standardize on JSON::MaybeXS
add our HTTP::Throwable subclasses to 00-compile.t
switch from Module::Load to Module::Runtime