This PR changes the way marshalling and unmarshalling behaves, in order to maximize compatibilty with big.Int: as in, maximize the chance that a 'drop in' replacement of big.Int for uint256.Int will work seamlessly.

• String() - before this change, String() would return the integer in hexadecimal format. This PR changes it to instead return the input in decimal format, like big.Int does.
• MarshalText() now returns the integer in decimal format, previously hexadecimal.
• MarshalJSON() now returns the integer in decimal format, previously hexadecimal.
• UnmarshalText now accepts either hex, 0x234 or 234. Previously it accepted only hex.
• UnmarshalJSON now accepts either hex-string, "0x234", dec-string "234" or naked numeric decimal 234. Previously it accepted only string-hex.

JSON marshalling is, alas, not 100% compatible, since big.Int marshals to json numeric format: { Foo: 5} as opposed to string-format: { Foo: "5" }. The former is not ideal for large numbers, since platforms like javascript do not support arbitary large numbers, usually capped at 53 bits or so.

To clarify a bit more, re backwards compatibility.

Example 1

If you previously had a struct,

type MyThing struct{
	Value *big.Int
}

And you json-encoded that to disk. the format would be e.g {"Value":5}. If you then replace big with uint256

type MyThing struct{
	Value *uint256.Int
}

Before this PR: The json-encoded old format could not be unmarshalled into the new uint256-format, since uint256.UnmarshalJSON only accepted hex-encoded strings.

After this PR: The json-encoded old format can be unmarshalled into the new uint256-format, since uint256.UnmarshalJSON now accepts json numerics.