vpar.gp tree functions

[Kevin Ryde <user42_kevin@yahoo.com.au>]

I used gp for some simple properties of trees and tried to collect a bit
of code into presentable form,

    http://user42.tuxfamily.org/pari-vpar/index.html

vpar is a fairly usual vertex parent array.  It's labelled and oriented,
though many of the properties calculated don't depend on that but are as
for "free" trees.

Not much is particularly gp specific, but it's convenient and ties in to
say factorizing, guessing recurrences, or verifying formulas or limits
on a sequence of tree sizes.

The functions are all or nearly all linear in the number of vertices so
can go to big individual trees, but overall speed is not enough for say
exhaustive searches of all trees.