experimental localprec() feature

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developpers,

I have created an experimental git branch bill-localprec that adds a command
localprec() which allow to set the real precision locally:

Consider the function:
f(s)=Pi^s
and
s=precision(1.,100)

Pi is computed using the current precision.
If we want the resut to be correct to the precision of s,
we could do

g(s)=
{ my(prec=default(realprecision));
  default(realprecision,precision(s));
  my(r=Pi^s);
  default(realprecision,prec);
  r;
}

which is rather cumbersome. Using localprec() it is sufficient to say:

g(s)=localprec(precision(s));Pi^s

Also it is safer since the value of realprecision is restored even if an error
occurs.

Cheers,
Bill.