[R-Source] Revise R AST model

  - Add nullable annotations
  - Fix SpecItem.getChild (value child)
  - Fix Sign.equalsValue
  - Fix DocuText.equalsSingle
  - Add type to .getRParent if applicable

Change-Id: If9e9be7c39c79094082c6656ec92b90d1f372340
40 files changed
tree: 66b790873e1df81e728a7619f0d615d127972581
  1. .project
  2. .settings/
  3. LICENSE
  4. NOTICE.md
  5. pom.xml
  6. r/
  7. redocs/
  8. rhelp/
  9. rtm/