Add QCBOREncode_Tell and QCBOREncode_SubString (#251)

* QCBOREncode_Tell and QCBOREncode_SubString

* Test and documentation improvements

* test, doc, back out error checks that didn't work

* more tests; tidiness

* Small documentation improvement

* Add RetreiveUndecodedInput

* Proper factoring for UsefulOutBuf Storage

* nits

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/test/run_tests.c b/test/run_tests.c
index d1d49ed..140eb53 100644
--- a/test/run_tests.c
+++ b/test/run_tests.c
@@ -162,6 +162,7 @@
     TEST_ENTRY(ExponentAndMantissaEncodeTests),
 #endif /* QCBOR_DISABLE_EXP_AND_MANTISSA */
     TEST_ENTRY(ParseEmptyMapInMapTest),
+    TEST_ENTRY(SubStringTest),
     TEST_ENTRY(BoolTest)
 };