Tidy up decode-related headers (#280)

Check and reduce some dependency between decode headers

Rename qcbor_decode to qcbor_main_decode to be more clear and help with header backwards compatibility 

Fix a header include backwards compatibility to v1 issue

Fix long lines and trailing spaces


Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index adffeeb..6ee40e3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,7 +39,7 @@
 target_sources(qcbor
     PRIVATE
         src/ieee754.c
-        src/qcbor_decode.c
+        src/qcbor_main_decode.c
         src/qcbor_spiffy_decode.c
         src/qcbor_tag_decode.c
         src/qcbor_number_decode.c