Make IssueState available as a field on Issue.

* Add `state` on `Issue` to be able to query the state of an issue even
  if you only have a reference to this issue and don't have a reference
  to the context or issues table. This does not change the persisted
  format of the issue. On disk the state of an issue is still
  represented by it's location in the file hierarchy.

* Refactored libpit to use zero_functional instead of sequtils.

src/pitpkg/cliconstants.nim

@@ -1,4 +1,4 @@
-const PIT_VERSION* = "4.22.2"
+const PIT_VERSION* = "4.23.0"
 
 const USAGE* = """Usage:
   pit ( new | add) <summary> [<state>] [options]