expose new container construction utilities

[mcol]

In stan-dev/math#1620 and stan-dev/math#1636 we added new utilities to create containers. The following signatures should be added to the language:

matrix identity_matrix(int)

int[] one_hot_int_array(int, int)
real[] one_hot_array(int, int)
row_vector one_hot_row_vector(int, int)
vector one_hot_vector(int, int)

int[] ones_int_array(int)
real[] ones_array(int)
row_vector ones_row_vector(int)
vector ones_vector(int)

real[] linspaced_array(int, real, real)
row_vector linspaced_row_vector(int, real, real)
vector linspaced_vector(int, real, real)

vector uniform_simplex(int)

int[] zeros_int_array(int)
real[] zeros_array(int)
row_vector zeros_row_vector(int)
vector zeros_vector(int)

[mcol]

This should wait for stan-dev/math#1666, as it adds a few more functions and renames some. I'll update the issue description when that's settled.

[mcol]

I've updated the description with the most recent changes, this is now ready to go.