experimental GP extensions

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Hello PARI developers,

I have created a GIT branch "bill-ranges" which add some experimental feature to GP
that we discussed at the Ateliers.

This is example of what is implemented:

ranges: [a..b] -> [a,a+1,...,b]
slices: V[a..b] -> [V[a],V[a+1],...,V[b]]
comprehension: [x^2+1 | x<-[1..40], isprime(x)] = apply(x->x^2+1,select(isprime,[1..40]))

Please experiment with the new features.

Cheers,
Bill.