Remove ASTNode::m_value member

Values are no more stored into AST nodes. The reason for that is that it was not
appropriate for multi-threaded evaluation of functions, since multiple
evaluations could not occur at the same time.

Now expression evaluation do not fill the node value but eventually return the
computed value.

Also, function are evaluated using a context array of values. Thus each function
calls is done in a separated context (as it should have always been).

This is a big change in the AST execution strategy. Nonetheless, it leads to a
seemingly better code, and there will be no more undefined trailing values in
the AST (for some node ASTNode::m_value would never be initialized/used).