returns the description of the coefficient field of a ring.
Example:
ring r=32003,(x,y),dp;
charstr(r);
==> 32003
ring s=0,(x,y),dp;
charstr(s);
==> 0
ring ra=(7,a),(x,y),dp;
minpoly=a^3+a+1;
charstr(ra);
==> 7,a
ring rp=(49,a),(x,y),dp;
charstr(rp);
==> 49,a
ring rr=real,x,dp;
charstr(rr);
==> real