Refactoring the Evaluator

[John W. Eaton]

I started refactoring the evaluator in this changeset:

  http://hg.savannah.gnu.org/hgweb/octave/rev/c452180ab672

As the commit message says, the idea is to use tree_evaluator class to 
walk the parse tree instead of rvalue methods in the parse tree classes. 
 To do that requires using a stack in the evaluator class to accumulate 
results.  Timing the test suite before and after this change show no 
significant change in performance.

There are still a number of places where evaluation is performed outside 
of the tree_evaluator class, primarily when generating lvalue objects 
and when evaluating functions.  For these operations, a pointer to the 
tree_evaluator object is required and it is either passed to the 
functions that need it or accessed globally.  Eventually, the global 
access should be eliminated.

Once all evaluation is handled in the tree_evaluator class, the parse 
tree object should just be a container object that holds a simple 
representation of Octave code.  At that point, I'd like to work toward a 
linear evaluator to replace the current tree walker evaluator.  The idea 
is to convert the parse tree to a linear list of instructions.  Then 
evaluation is a loop iteration instead of chasing pointers in a tree. 
Having the linear representation may also aid in interfacing with a JIT 
compiler.  As I recall, Max did something like this already when 
prototyping the LLVM JIT compiler (minus the evaluator; he went straight 
from the linear represenation to LLVM).  So maybe some of what he did 
can be reused.

jwe