RelBuilder.project should combine expressions if underlying node is a Project

The RelBuilder.project method should combine expressions if underlying node is a Project.

For example, if r is a RelBuilder and its stack initially contains

Project(Scan(T), a, b, a + b as c)

and we call (in pseudo-code)

r.project(a + c as d)

then the result should be

Project(Scan(T), a + (a + b) as d)

Today the result is

Project(Project(Scan(T), a, a + b as c), a + c as d)

In some circumstances the result will be larger (i.e. contain more RexNode instances) but much more frequently the result will be smaller and simpler.