Refine SAVE and GUARD_OBJ macros

[nobu]

Get rid of risk of vStack overflow.
As a trade off:

• the argument of SAVE must be a variable
• multiple GUARD_OBJ cannot be placed at one line

[tompng]

I'm planning to remove obj field from struct Real in #344.
This back pointer is the main reason why GUARD_OBJ is needed.

typedef struct {
    VALUE  obj;     /* Back pointer(VALUE) for Ruby object.   */
    size_t MaxPrec;
    size_t Prec;
    SIGNED_VALUE exponent;
    short  sign;
    short  flag;
    DECDIG frac[FLEXIBLE_ARRAY_SIZE];
} Real;

After #344, most GUARD_OBJ can be removed, and we only need few RB_GC_GUARD, I think.