Allow underscores in integer and numeric constants.
 
This allows underscores to be used in integer and numeric literals,
and their corresponding type input functions, for visual grouping.
For example:
 
    1_500_000_000
    3.14159_26535_89793
    0xffff_ffff
    0b_1001_0001
 
A single underscore is allowed between any 2 digits, or immediately
after the base prefix indicator of non-decimal integers, per SQL:202x
draft.
 
Peter Eisentraut and Dean Rasheed
 
Discussion: https://postgr.es/m/84aae844-dc55-a4be-86d9-4f0fa405cc97%40enterprisedb.com