Use node iterators

[CrockAgile]

To close #13 , the internal vectors used to hold productions, expressions, and terms are no longer public. For users to still be able to iterate and modify these types, the following public functions have been added:

// bnf::node

Expression::add_term(&mut self, Term)
Expression::remove_term(&mut self, &Term) -> Option<Term>
Expression::terms_iter(&self) -> Expression::Iter
Expression::terms_iter_mut(&mut self) -> Expression::IterMut

Production::add_to_rhs(&mut self, Expression)
Production::remove_from_rhs(mut self, &Expression) -> Option<Expression>
Production::rhs_iter(&self) -> Production::Iter
Production::rhs_iter_mut(&mut self) -> Production::IterMut

Grammar::add_production(&mut self, Production)
Grammar::remove_production(&mut self, &Production) -> Option<Production>
Grammar::productions_iter(&self) -> Grammar::Iter
Grammar::productions_iter_mut(&mut self) -> Grammar::IterMut

bnf::node was also reorganized into a directory with respective grammar/production/expression/term files.

[coveralls]

Coverage increased (+8.2%) to 92.089% when pulling b28b2ad712f6467cafa79e764fd565c3fd299a61 on use-node-iterators into 21338d3b66f6876b7822545b2225ed3399268473 on 0.2.0-devel.

[shnewto]

👏👏👏

Awesome add, solid testing and it all looks great! Thanks @CrockAgile !!