u3: refactor cttp, http, lord, and newt modules to better share functionality

[mcevoypeter]

Reduce duplicated code in the following ways:

• Factor out functionality shared between cttp and http modules into a shared module that both can use without duplicating code.
• Replace the in-place linked list implementation in u3_hhed with c3_list.
• Replace the in-place linked list implementation in u3_lord with c3_list.
• Pull newt interface into its own header file in preparation for replacing in-place linked list implementations in newt data structures.

This is an ongoing process, but I'm putting this chunk of work up for review now so that the diff remains manageable for other eyes.

Test plan:

$ urbit -F zod
$ valgrind urbit zod

No new memory leaks or errors were reported when running under valgrind.