cast from constant poly to bigint

[gorzelc@…]

The casts from a constant poly to int and number are implemented,

but the corresponding cast to bigint is missing,

       > ring r=0,x,dp;

       > poly f = 12;
       > int(f);    // OK
       12
       > number(f);  // OK
       12
       > bigint(f);
	  ? bigint(`poly`) failed
	  ? expected bigint(`bigint`)
	  ? expected bigint(`number`)
	  ? error occurred in STDIN line 14: `bigint(f);`

[hannes]

bigint(poly-expr) for const polys.