Added cbegin() and cend() to most containers

[dcdillon]

This PR should serve as the baseline for implementation of cbegin()/cend() functions on Rcpp containers. It preserves functionality as-is for most things and is simply an addition of new functions, but it also sets the table for getting const correctness across the board.

Conspicuously missing is MatrixRow because I forgot and we already ran extensive testing on what's here. This will be addressed in a future PR.

Fundamentally, we did two things:

1. Created the concept of both iterator and const_iterator within VectorBase. It always had iterator but it wasn't really an iterator. It was called iterator and behaved like a const_iterator except that instead of returning const & it returned items by value on dereferencing. This functionality has been preserved because returning & and const & causes issues with the various proxy classes. This will need to be addressed in a future PR. Currently VectorBase only returns
   const_iterator from the begin(), end(), cbegin(), cend() functions which is consistent with previous behavior.
2. Added cbegin()/cend() function to Vector and its derivatives. This is consistent with current behavior, but does not change whether the const_iterator is actually const in the case of proxy classes (specifically const_string_proxy which can be assigned to against all reason). This too will be changed in a future PR.