Add lib.yang.list hash trie to store YANG lists

[eugeneia]

This adds a hash trie implementation to be used for storing all YANG lists. The motivation for this is twofold:

• before YANG lists could be represented as three different data structures: lib.ctable, lib.cltable, and native Lua tables. Ctable and cltable representations are important optimizations as they can store huge configurations (think routing tables) efficiently. With this PR all lists are represented as lib.yang.list hash tries, which should give both the efficiency of a specialized data structure and the ease of access of native tables via the use of metatables.
• a downside of ctables and cltables was that they are unordered collections. YANG however specifies the ordered-by statement for lists. lib.yang.list tries are ordered and hence support this statement.

Implementation wise this PR:

• enables LUA52COMPAT to provide full operator overloading for lib.yang.list
• enabling LUA52COMPAT uncovered some unrealiability in StackTracePlus. I’ve checked in the latest version, but we still need to fallback to debug.traceback() sometimes.

Functionality wise this PR affects:

• Snabb lwAFTR: we are aiming for feature parity. There is a new extra step when loading the initial configuration where the YANG list is compiled to a ctable binding table. Data-plane behavior should be unchanged.
• Snabbflow (snabb ipfix probe): flow director classes in schema use ordered-by user, and leaves referring to other key leaves now use the leafref type.
• Future users of lib.yang and lib.ptree: more types of configuration schemas should be able to be represented efficiently now. lib.yang.list stores strings off-heap, so huge lists with string keys or members should be more ecomonical.

Todo:

• tests pass
• benchmarks should run and not have degraded in performance