First, if the ziplist header is surely inside the ziplist, do fast path
decoding rather than the careful one.
In that case, streamline the encoding if-else chain to be executed only
once, and the encoding validity tested at the end.
encourage inlining
likely / unlikely hints for speculative execution
Assertion used _exit(1) to tell the compiler that the code after them is
not reachable and get rid of warnings.
But in some cases assertions are placed inside tight loops, and any
piece of code in them can slow down execution (code cache and other
reasons), instead using either abort() or better yet, unreachable
builtin.
This header file is for libs, like ziplist.c, that we want to leave
almost separted from the core. The panic() calls will be easy to delete
in order to use such files outside, but the debugging info we gain are
very valuable compared to simple assertions where it is not possible to
print debugging info.
