lib/diag.rs refactor

This splits diag.rs, which was growing way too big for my taste, into a
number of submodules. This should help us compartmentalize tests better,
as well as use mod namespaces to shorten our struct/enum names.

lib/tests/test_lte_parsing.rs

@@ -1,6 +1,7 @@
 use deku::prelude::*;
 use rayhunter::{
-    diag::{LogBody, LteRrcOtaPacket, Message, Timestamp},
+    diag::Message,
+    diag::diaglog::{LogBody, rrc::LteRrcOtaPacket, Timestamp},
     gsmtap_parser,
 };