Add Ecuadorian CI and RUC numbers

Add modules for Ecuadorian Identification Card (CI - Cédula de
identidad) and Fiscal Numbers (RUC - Registro Único de Contribuyentes)
Use dedicated doctests

This moves a number of the existing test cases to dedicated doctest
files and extend the tests with more numbers and corner cases.

This also fixes a few docstrings.
Refactor checksum functions

Use the CI checks from within the RUC module for natural RUC numbers
(thereby eliminating a bug in the RUC checksum calculation) and simplify
the checksum functions.
Validate parts of numbers

This raises exceptions when the provice or establishment number part of
the number contains invalid values.
Add Ecuadorian CI and RUC numbers

Add modules for Ecuadorian Identification Card (CI - Cédula de
identidad) and Fiscal Numbers (RUC - Registro Único de Contribuyentes)

See: https://github.com/arthurdejong/python-stdnum/pull/12