* include/lightning.h, include/lightning/jit_private.h,
lib/lightning.c: Implement the new jit_set_code() interface,
that allows instructing lightning to use an alternate code
buffer. The new jit_realize() function should be called
before jit_set_code(), and usually call jit_get_code()
to query the amount of bytes expected to be required for
the code.
* lib/jit_size.c: Minor update to have less chances of
miscalculating the code buffer by starting the counter
with the size of the longest instruction instead of zero,
as code emit fails if at any moment less than the longest
instruction bytes are available.
* check/setcode.c: New file implementing some basic tests
of the new jit_set_code() interface.
* check/Makefile.am: Update for newer test case.
