typval_encode: Do not use new vector type

For string() it looks like no optimization, sometimes performance is even worse. 
Since it was designed to avoid heap allocations in clear_tv let’s see whether it 
will make any difference once clear_tv uses typval_encode to avoid stack 
overflow in the disabled test.

src/nvim/eval/typval_encode.h

@@ -168,13 +168,13 @@ typedef struct {
 } MPConvStackVal;
 
 /// Stack used to convert VimL values to messagepack.
-typedef kvec_withinit_t(MPConvStackVal, 8) MPConvStack;
+typedef kvec_t(MPConvStackVal) MPConvStack;
 
 // Defines for MPConvStack
 #define _mp_size kv_size
-#define _mp_init kvi_init
-#define _mp_destroy kvi_destroy
-#define _mp_push kvi_push
+#define _mp_init kv_init
+#define _mp_destroy kv_destroy
+#define _mp_push kv_push
 #define _mp_pop kv_pop
 #define _mp_last kv_last