version of array_builder producing matrix and row_vector objects without copies

Summary:

Right now, the usage through Stan in the generated code is to construct a std::vector, fill it, then copy it, then convert that into a matrix or row vector. The intermediate copy should be eliminated to reduce memory contention on the heap from the redundant copy.

This should then be pushed through the generator code in Stan for row vector and matrix expressions.

Description

The new method should look like this:

Eigen::Matrix<T, -1, 1> array_builder<T>::row_vector()

and similarly for ::matrix().

This will require either writing a new array-builder class inside the matrix directory or writing a new matrix_builder and new row_vector_builder class. These are still going to require std::vector on the inside to be able to incrementally build a sequence without knowing its size ahead of time.

Current Version:

v2.14.0