Better error for _EnterBstrWrapped on allocated (indef) strings (#221)

Better error for QCBORDecode_EnterBstrWrapped on allocated strings. Since indefinite-length strings are always allocated, this gives a better error for attempts to use this on indefinite-length strings.

The TODO: to make this work is closed out as it was decided not to support QCBORDecode_EnterBstrWrapped() on indefinite-length strings. To do so would require increasing the size of QCBORItem.

* Add limitations comment

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/inc/qcbor/qcbor_common.h b/inc/qcbor/qcbor_common.h
index 288c691..752d149 100644
--- a/inc/qcbor/qcbor_common.h
+++ b/inc/qcbor/qcbor_common.h
@@ -523,6 +523,11 @@
     * (to save object code). */
    QCBOR_ERR_RECOVERABLE_BAD_TAG_CONTENT = 78,
 
+   /** QCBORDecode_EnterBstrWrapped() cannot be used on
+    * indefinite-length strings because they exist in memory pool for
+    * a @ref QCBORStringAllocate. */
+   QCBOR_ERR_CANNOT_ENTER_ALLOCATED_STRING = 79,
+
    /** A range of error codes that can be made use of by the
     * caller. QCBOR internally does nothing with these except notice
     * that they are not QCBOR_SUCCESS. See QCBORDecode_SetError(). */
diff --git a/inc/qcbor/qcbor_encode.h b/inc/qcbor/qcbor_encode.h
index 75b4d64..774f272 100644
--- a/inc/qcbor/qcbor_encode.h
+++ b/inc/qcbor/qcbor_encode.h
@@ -366,6 +366,7 @@
  * - Tags on labels are ignored during decoding.
  * - The maximum tag nesting is @c QCBOR_MAX_TAGS_PER_ITEM (typically 4).
  * - Works only on 32- and 64-bit CPUs.
+ * - QCBORDecode_EnterBstrWrapped() doesn't work on indefinite-length strings.
  *
  * The public interface uses @c size_t for all lengths. Internally the
  * implementation uses 32-bit lengths by design to use less memory and
diff --git a/inc/qcbor/qcbor_spiffy_decode.h b/inc/qcbor/qcbor_spiffy_decode.h
index 32a8ef5..b374e9e 100644
--- a/inc/qcbor/qcbor_spiffy_decode.h
+++ b/inc/qcbor/qcbor_spiffy_decode.h
@@ -1780,8 +1780,8 @@
  * CBOR. QCBORDecode_ExitBstrWrapped() must be called to resume
  * processing CBOR outside the wrapped CBOR.
  *
- * This does not (currently) work on indefinite-length strings. The
- * (confusing) error @ref QCBOR_ERR_INPUT_TOO_LARGE will be set.
+ * This does not work on indefinite-length strings. The
+ * error @ref QCBOR_ERR_CANNOT_ENTER_ALLOCATED_STRING will be set.
  *
  * If @c pBstr is not @c NULL the pointer and length of the wrapped
  * CBOR will be returned. This is usually not needed, but sometimes