cameo: adding support for dril/mill substitutions (in progress)

- Makefile (OBJS): added shape.o
- README: added section for drill/mill substitutions
- lang.l, lang.y: added commands "drill", "empty", "mill", "remainder", and
  "reset"
- lang.y (clear_paths): moved freeing of all paths from "clean" to shared
  function clear_paths
- ops.h, ops.c (try_drill, try_mill): helper functions for "drill" and
  "mill" commands
- shape.h. shape.c (slot, circle): functions to generate toolpaths for
  basic shapes

cameo/README

@@ -136,3 +136,11 @@ Concave corners on an outside path are normally cut such that the corner
 is round, leaving material at and near the corner point. The option "dog"
 changes this to cutting a "dogbone" hole such that material is also
 removed up to the corner point.
+
+
+Drill/mill substitution:
+
+  drill <min-diameter> <max-diameter>
+  mill <diameter> <step>
+  remainder
+  empty