bench.lua refactors

* deps/pflua: Update.
* src/bench.lua: Remove dlopen shenanigans and just use Lua FFI API.
Refactor kernel JIT / bench.lua integration

* src/bench.lua: Pass a sock_fprog directly to the Linux JIT without
  doing the horrible string thing.

* src/ref/bpf-jit-kernel/bpf_jit_comp.c (bpf_alloc_binary): Use mmap to
  allocate memory dynamically instead of statically using a two-page
  buffer.

* src/ref/bpf-jit-kernel/bpf_jit_kernel.c:
* src/ref/bpf-jit-kernel/bpf_jit_kernel.h: Remove a lot of the old
  string code.
bench.lua no longer disables gc

* src/bench.lua: Don't stop GC around tests.