fix: jv_number_value should cache the double value of literal numbers

The code of `jv_number_value` is intended to cache the double value of
literal numbers, but it does not work because the function accepts the
`jv` struct by value. This patch fixes the behavior by checking if the
double value is `NaN`, which is the initial value. This patch apparently
does not cache the value if the literal is `NaN`, but improves the
performance of major use cases; e.g. `range(1000000)` runs 25% faster.

src/jv.c

@@ -206,9 +206,6 @@ enum {
   JVP_NUMBER_DECIMAL = 1
 };
 
-#define JV_NUMBER_SIZE_INIT      (0)
-#define JV_NUMBER_SIZE_CONVERTED (1)
-
 #define JVP_FLAGS_NUMBER_NATIVE       JVP_MAKE_FLAGS(JV_KIND_NUMBER, JVP_MAKE_PFLAGS(JVP_NUMBER_NATIVE, 0))
 #define JVP_FLAGS_NUMBER_LITERAL      JVP_MAKE_FLAGS(JV_KIND_NUMBER, JVP_MAKE_PFLAGS(JVP_NUMBER_DECIMAL, 1))
 
@@ -590,7 +587,7 @@ static jv jvp_literal_number_new(const char * literal) {
     return JV_INVALID;
   }
 
-  jv r = {JVP_FLAGS_NUMBER_LITERAL, 0, 0, JV_NUMBER_SIZE_INIT, {&n->refcnt}};
+  jv r = {JVP_FLAGS_NUMBER_LITERAL, 0, 0, 0, {&n->refcnt}};
   return r;
 }
 
@@ -698,9 +695,8 @@ double jv_number_value(jv j) {
   if (JVP_HAS_FLAGS(j, JVP_FLAGS_NUMBER_LITERAL)) {
     jvp_literal_number* n = jvp_literal_number_ptr(j);
 
-    if (j.size != JV_NUMBER_SIZE_CONVERTED) {
+    if (isnan(n->num_double)) {
       n->num_double = jvp_literal_number_to_double(j);
-      j.size = JV_NUMBER_SIZE_CONVERTED;
     }
 
     return n->num_double;
@@ -731,7 +727,7 @@ int jvp_number_is_nan(jv n) {
     return decNumberIsNaN(pdec);
   }
 #endif
-  return n.u.number != n.u.number;
+  return isnan(n.u.number);
 }
 
 int jvp_number_cmp(jv a, jv b) {