Add MRB_WORD_BOXING mode (represent mrb_value as a word)

### Summary
MRB_WORD_BOXING mode represent mrb_value as a word (the natural data unit of the processor; 32 or 64 bit.) like ruby's VALUE.

### Changes in MRB_WORD_BOXING mode
- Fixnum --- stored as (<<1)ed value and LSB is set.
- False, Nil, True and Undef --- stored as MRB_Qfalse, MRB_Qnil, MRB_Qtrue, MRB_Qundef value
- Symbol --- stored as (<<8)ed value and lowest-byte is set to MRB_SYMBOL_FLAG.
- Float and VOIDP --- stored as RFloat/RVoidp object (now, they are managed by GC)