VMODs can now use the same facility as native types to find methods
based on the type of the instance instead of the instance itself.

Because SYM_METHOD symbols are tied to a type instead of an instance
we lose that "extra" argument. To circumvent this, an expression will
keep track of the instance before evaluating a method.

With this, VCL expression evaluation is no longer aware of methods
implementation details, and feeds entirely from the symbol table.