box: move info_handler interface into src/info

Box/info.h defines info_handler interface with a set
of virtual functions. It allows to hide Lua from code
not depending on this language, and is used in things
like index:info(), box.info() to build Lua table with
some info. But it does not depend on box/ so move it
to src/.

Also, this API is needed for the forthcoming SWIM
module which is going to be placed into src/lib and
needs info to dump its state to Lua from C without
strict Lua dependency.

@locker:
 - remove pointless _GNU_SOURCE definition from
   box/lua/info.c
 - remove luaT_info_handler_create declaration from
   box/lua/info.h

Needed for #3234