py/misc: Remove use of bitfield from vstr_t.

Since there is only one flag, we don't need to use a bitfield in vstr_t.
Compilers emit extra instructions to access a bitfield, so this should
reduce the binary size a small amount.

Signed-off-by: David Lechner <david@pybricks.com>

py/misc.h

@@ -179,7 +179,7 @@ typedef struct _vstr_t {
     size_t alloc;
     size_t len;
     char *buf;
-    bool fixed_buf : 1;
+    bool fixed_buf;
 } vstr_t;
 
 // convenience macro to declare a vstr with a fixed size buffer on the stack