Add GetSimple, update Simple Value documentation (#236)

Add the method QCBORDecode_GetSimple().  Update testing and documentation for simple values.


* Add GetSimple

* Finish off GetSimple and friends

* Fix tests when tags are disabled

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/test/qcbor_decode_tests.h b/test/qcbor_decode_tests.h
index b23149a..a08a3af 100644
--- a/test/qcbor_decode_tests.h
+++ b/test/qcbor_decode_tests.h
@@ -126,7 +126,7 @@
 /*
  Test parsing of some simple values like true, false, null...
  */
-int32_t ParseSimpleTest(void);
+int32_t SimpleValueDecodeTests(void);
 
 
 /*