The new API is (roughly) as discussed in
  #451

This is quite a large commit and it is not convenient to split it up.
It contains the following changes:

 * Redo the list builder and accessor macros implemnetation,
   including docs and tests.

 * Change uses of define_list_config_builder.  In each case:
   - Move the docs about the default value to the containing field.
   - Remove the other docs (which were just recapitulations, and
     are now not needed since the ListBuilder is no longer public).
   - Rewmove or replace `pub` in the define_list_builder_helper call,
     so that the builder is no longer public.
   - Change the main macro call site to use define_list_builder_helper.
   - Add a call to define_list_builder_accessors.

 * Make the module `list_builder` pub so that we have somewhere to
   put the overview documentation.

 * Consequential changes:
   - Change `outer.inner().replace(X)` to `outer.set_inner(X)`
   - Consequential changes to imports (`use` statements).