This adds full support for formatting a uint256 along with associated
tests to ensure proper functionality.
It includes a fmt.Formatter that supports the full suite of the fmt
package format flags for integral types, a fmt.Stringer, and a separate
Text method that accepts an output base directly and produces the
relevant output with fewer allocations than using the standard fmt
methods.
This is part of a series of commits to fully implement the uint256
package.