libwebsockets

lwsac: add blob deduplication helper

Add lwsac helper api to allow user code to perform constant string folding
easily within an lwsac.  After isolating a string or blob that it wants to store in
the lwsac and point to, it can check if the string or blob already exists earlier
in the lwsac first, and if so just point to that without copying it in again.

For some formats with repeated strings like JSON, the saving can add up to
something useful.