Rebase VLE code

Rebased the VLE code -

Pulled out the graphid list and stack data structures used by the
VLE so that they may be used by others. This also provides a good
place for other graph related data structures to be stored.

Pulled out the graph creation and management routines and the graph
vertex and edge data structures to allow for other uses beyond VLE.

The data structures for the graph were purposely not moved in with
the other data structures as they are vital to the graph creation
and management logic. It might be useful to eventually refactor this
to expand what a graph may contain. But, for now, it was left alone.

The header files: age_global_graph.h and age_graphid_ds.h have the
interfaces for these functions and are designed to hide the
implementation from the user.

All regression tests completed without issue.

Makefile

@@ -48,12 +48,14 @@ OBJS = src/backend/age.o \
        src/backend/parser/cypher_parse_agg.o \
        src/backend/parser/cypher_parse_node.o \
        src/backend/parser/cypher_parser.o \
+       src/backend/utils/adt/age_graphid_ds.o \
        src/backend/utils/adt/agtype.o \
        src/backend/utils/adt/agtype_ext.o \
        src/backend/utils/adt/agtype_ops.o \
        src/backend/utils/adt/agtype_parser.o \
        src/backend/utils/adt/agtype_util.o \
-       src/backend/utils/adt/agtype_vle.o \
+       src/backend/utils/adt/age_global_graph.o \
+       src/backend/utils/adt/age_vle.o \
        src/backend/utils/adt/cypher_funcs.o \
        src/backend/utils/adt/ag_float8_supp.o \
        src/backend/utils/adt/graphid.o \