Add QCBORDecoder_Tell(); traversal cursor position bug fix (#223)

Adds QCBORDecode_Tell() to return traversal cursor position.

Fixes a traversal cursor position bug. The position was not correct after a QCBORDecode_GetXXXInX() of a non-aggregate item by label (e.g., QCBORDecode_GetInt64InMapN()) in an array that was entered.

Fixes to documentation for traversal cursor.

Addresses #213


* Add QCBORDecoder_Tell()

* Testing near complete and passing

* More tests and documentation

* Add some more documentation

* typo

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/test/qcbor_decode_tests.h b/test/qcbor_decode_tests.h
index 0cedf43..8d26c98 100644
--- a/test/qcbor_decode_tests.h
+++ b/test/qcbor_decode_tests.h
@@ -323,4 +323,7 @@
 int32_t ErrorHandlingTests(void);
 
 
+int32_t TellTests(void);
+
+
 #endif /* defined(__QCBOR__qcbort_decode_tests__) */