Added text support in DRC

- A new constructor for RBA::Region has been provided to
  create text markers from a shape iterator
- The DRC framework has been enhanced with a "text" function

testdata/drc/drctest.drc

@@ -497,6 +497,9 @@ cell("TOP")
 
 c1.rounded_corners(0.5, 0.5, 16).output(1010, 0)
 c1.smoothed(1.5).output(1011, 0)
+a1.texts.output(1020, 0)
+a1.texts("A*").output(1021, 0)
+a1.texts("A*", false).output(1022, 0)
 
 puts "=== Single-cell testsuite ==="