Remove has_source_hash from iseq_body

We can use a special value of 0 in source_hash to denote that it does not
have source_hash. This removes the has_source_hash field which saves us 8
bytes per iseq.

ruby_parser.c

@@ -1119,7 +1119,8 @@ rb_source_hash_update(rb_source_hash_state_t *state, const uint8_t *ptr, size_t
 uint64_t
 rb_source_hash_finalize(const rb_source_hash_state_t *state)
 {
-    return state->hash;
+    /* A hash of 0 means no source hash, so remap it to another value. */
+    return state->hash == 0 ? 1 : state->hash;
 }
 
 VALUE