lib: serialize MAC RACH attempts to GSMTAP

This also refactors the gsmtap code into a neater module, and adds MAC
UL & DL logs to our diag capture.

lib/src/test_util.rs

@@ -0,0 +1,11 @@
+use deku::reader::Reader;
+use std::io::Cursor;
+
+pub fn unhexlify(hexlified_bytes: &str) -> (usize, Reader<Cursor<Vec<u8>>>) {
+    let byte_len = hexlified_bytes.len() / 2;
+    let bytes = (0..hexlified_bytes.len())
+        .step_by(2)
+        .map(|i| u8::from_str_radix(&hexlified_bytes[i..i + 2], 16).unwrap())
+        .collect();
+    (byte_len, Reader::new(Cursor::new(bytes)))
+}