Support for grabbing multiple consecutive values with nextval()

Hi,

First time PostgreSQL contributor here :)

I wanted to be able to allocate a bunch of numbers from a sequence at 
once. Multiple people seem to be struggling with this 
(https://stackoverflow.com/questions/896274/select-multiple-ids-from-a-postgresql-sequence, 
https://www.depesz.com/2008/03/20/getting-multiple-values-from-sequences/).

I propose to add an extra argument to nextval() that specifies how many 
numbers you want to allocate (default 1).

The attached patch (based on master) passes `./configure 
--enable-cassert --enable-debug && make && make check`, including the 
newly added regression tests.

It does change the signature of nextval_internal(), not sure if that's 
considered backwards compatibility breaking (for extensions?).

-- Jille