Move tag-related and spiffy-related code to their own source files

qcbor_decode.c was just too big

* Move all tag-related code into qcbor_tag_decode.[ch]

* split out spiffy decode source and DecodeNesting

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c37cb38..adffeeb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,6 +40,7 @@
     PRIVATE
         src/ieee754.c
         src/qcbor_decode.c
+        src/qcbor_spiffy_decode.c
         src/qcbor_tag_decode.c
         src/qcbor_number_decode.c
         src/qcbor_encode.c