Extract a CBB helper

This was nagging at me, so I thought I'd see what I could do. I'm not
worried about pinning now - the CBB by inspection is definitely
movable, and it would need to be altered in fairly weird ways.

But I am worried about the lifetime issues - that seems like an
accident waiting to happen, but can be exposed to the borrow checker
fairly easily.

(This is similar to commit 3194c466151c7d1c06b226f13d3c03298a27d40a.)

Bug: 299055662
Test: TH
Change-Id: Iada56f51800cd2af99e86b557805e65823a141da
diff --git a/rialto/src/requests/mod.rs b/rialto/src/requests/mod.rs
index 8162237..12838f3 100644
--- a/rialto/src/requests/mod.rs
+++ b/rialto/src/requests/mod.rs
@@ -15,6 +15,7 @@
 //! This module contains functions for the request processing.
 
 mod api;
+mod cbb;
 mod ec_key;
 mod pub_key;
 mod rkp;