Add explode/implode jq functions to match jv API

builtin.c

@@ -493,6 +493,8 @@ static const struct cfunction function_list[] = {
   {(cfunction_ptr)f_tonumber, "tonumber", 1},
   {(cfunction_ptr)f_tostring, "tostring", 1},
   {(cfunction_ptr)f_keys, "keys", 1},
+  {(cfunction_ptr)jv_string_explode, "explode", 1},
+  {(cfunction_ptr)jv_string_implode, "implode", 1},
   {(cfunction_ptr)jv_setpath, "setpath", 3}, // FIXME typechecking
   {(cfunction_ptr)jv_getpath, "getpath", 2},
   {(cfunction_ptr)jv_delpaths, "delpaths", 2},

jv.c

@@ -618,7 +618,7 @@ jv jv_string_implode(jv j) {
   assert(len >= 0);
 
   for (i = 0; i < len; i++) {
-    jv n = jv_array_get(j, i);
+    jv n = jv_array_get(jv_copy(j), i);
     assert(jv_get_kind(n) == JV_KIND_NUMBER);
     s = jv_string_append_codepoint(s, jv_number_value(n));
   }