New GP function self() for recursive anonymous functions

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

Dear PARI developers,

I have added a GP function self() which allow to write recursive anonymous
functions:

? (n->if(n==0,1,n*self()(n-1)))(5)
%1 = 120

self() returns the calling function or closure.

This is useful to avoid using global variables when using parallel GP:
For example
my(f(n)=if(n==0,1,n*self()(n-1))); parapply(f,[100..200])
is valid but
f(n)=if(n==0,1,n*f()(n-1))
parapply(f,[100..200])

is not.

Please tell me if it is useful to you.

Cheers,
Bill.