We have introduced following new instructions.

 * `OP_LAMBDA16`
 * `OP_BLOCK16`
 * `OP_METHOD16`
 * `OP_EXEC16`

Each instruction uses 16 bits operand for `reps` index. Since new
instructions are added, `mruby/c` VM should be updated.

Due to new instructions, dump format compatibility is lost, we have
increased `RITE_BINARY_MAJOR_VER`.

In addition, we have decreased the size of `refcnt` in `mrb_irep` from
`uint32_t` to `uint16_t`, which is reasonably big enough.