perf: add HEDLEY_PURE to o_val and o_len accessors

o_val and o_len are stateless field accessors with no side effects.
Annotating them as HEDLEY_PURE lets the compiler CSE repeated calls
in hot loops, particularly o_len in ZK witness generation paths.

Author: jaromil

src/zen_octet.c

@@ -369,8 +369,8 @@ octet *o_push(lua_State *L, const char *buf, size_t len) {
 
 // extern const char *o_val(const octet*);
 // extern size_t o_len(const octet*);
-const char *o_val(const octet *o) { return((const char*)o->val); }
-size_t o_len(const octet *o) { return(o->len); }
+HEDLEY_PURE const char *o_val(const octet *o) { return((const char*)o->val); }
+HEDLEY_PURE size_t o_len(const octet *o) { return(o->len); }
 
 void push_buffer_to_octet(lua_State *L, char *p, size_t len) {
 	octet* o = o_new(L, len); SAFEV(o, CREATE_OCT_ERR);