Efficient parameterizaton of ScmNumberFormat through ScmWriteControls

[shirok]

ScmNumberFormat allows to customize details of number to string and vice versa. From REPL, the user can change a few parameters of ScmNumberFormat via write-controls.

As we expose more parametres, it'd be tedious to mirror them in write-controls, and transfer them to ScmNumberFormat every time the number needs to be written. Rather, we may embed ScmNumberFormat in ScmWriteControls, and its getter/setter takes care of conversions. Then, we can just pass the pointer into ScmWriteControls in Scm__WritePrimitive() when we call Scm_PrintNumber.