TextMarshaller and TextUnmarshaller to ease transport of
unlimited precision rational numbers.

Fixes #7287.

Consists of encode and decode functions and two test
functions, one using JSON and one using XML. Each
verifies round trips for integers (rationals with
denominator == 1) and for fractional vaues.

LGTM=gri
R=gri, cookieo9, bradfitz, mtj
CC=golang-codereviews
https://golang.org/cl/61180043