Add string buffer macro and tests for Str8

src/core/strings/str8/str8.h

@@ -14,6 +14,8 @@ struct str8 {
   u8 *buf;
 };
 
+#define wapp_str8_buf(CAPACITY) ((Str8){.capacity = CAPACITY, .size = 0, .buf = (u8[CAPACITY]){0}})
+
 Str8 wapp_str8_lit(char *str);
 char wapp_str8_get(const Str8 *str, u64 index);
 void wapp_str8_set(Str8 *str, u64 index, char c);