Optimize code by merging expressions

Now that we are using Eigen expressions in Stan Math it makes sense for compiler to merge multiple expressions into singe one where possible to avoid unnecessary memory accesses. This is possible where we have a variable of matrix, vector or row_vector type (or if using OpenCL any type that is replaced with a matrix_cl) that is used in exactly one place in further code. Such a variable can be removed and its use case replaced with the expression that was assigned to the variable.

For example from stan code:

matrix a, b;
matrix c = a + b;
matrix d = c * 3;

We want to generate (assuming c is not used anywhere else):

Eigen::MatrixXd a, b;
Eigen::MatrixXd d = (a + b) * 3;

instead of:

Eigen::MatrixXd a, b;
Eigen::MatrixXd c = a + b;
Eigen::MatrixXd d = c * 3;

Start at the Stan-to-C++ expression-generation path illustrated by the matrix, vector, and row_vector examples, and trace how single-use variables are represented. Done means eligible assignments are merged into later expressions to reduce memory accesses, while variables used in multiple places remain available.