Adds EL_TRUE/EL_FALSE convenience macros to el_runtime.h alongside the
existing EL_NULL, making boolean-returning builtins readable without
raw (el_val_t) casts. Documents all value macros in the header comment.
Also lands el_arena_push/el_arena_pop — a scoped string arena for CLI
programs that never call el_request_start/end. The compiler can push a
mark before a compilation unit and pop it after to free intermediate
strings, reducing peak RSS during long compile runs.
Will Anderson