Add `CDL_CONCAT`

[Quuxplusone]

Reviewing old PRs, I noticed #45, from the year 2014, which wanted to add several new APIs. Of those:

• LL_POP_BACK, DL_POP_BACK, CDL_POP_BACK remain absent as of 2026.
• CDL_APPEND was added in API compatible extensions to functionality of UTList macros #67.
• CDL_CONCAT remains absent as of 2026.

I think the hardest thing about the _POP_BACK family of functions would be coming up with good names: _POP_BACK matches the C++ STL, but _APPEND doesn't; is there a better verb for "the opposite of append"?

But adding CDL_CONCAT on its own — filling in the one conspicuously empty slot in the "List Operations" table — should be a slam dunk. If anyone wants to take that on, be my guest.