Conversion utilities

[0xsimulacra]

Hello,

This pull request includes the following implementations and changes:

• Upgraded uint256 to its latest version (v1.2.4)
• FromDecimal() to parse a int256 in the form of "-1123" or "1235"
• Dec() will transform to a string to its decimal refresentation "1234" or "-234"
• String() now internaly use Dec() method
• FromHex() o parse a int256 in the form of "0x12fe" or "-0x23fd"
• ToHex() will transform to a string in the form of "0x.." or "-0x.."
• MarshalTextand UnmarshalText methods: implements encoding.TextMarshaler & encoding.TextUnmarshaler interfaces.
• MarshalJSON and UnmarshalJSON methods: to implements json.Marshaler & json.Unmarshaler interfaces, so we can encode a int256 to its json representation and we can decode it from a json.
• Test have been included for almost all the newly added methods (including a fuzz test for the FromDecimal methods that helped catch an implemntation error: when a string start with two or more signs ("--", "-+"..) it should produce an error same as in big.Int)
• MostSignificantBit() return the most significant bit of the abs part of z, so ignoring the bit or the sign
• Added Three usage examples in the read me, One of them being an almost finshed version of UniswapV3 GetTickAtSqrtRatio and another one is a Json Marshaling/UnMarshaling example.

Regarding the following fucniton that I added, maybe you don't want to have it and have only a BitLen() instead ?

// MostSignificantBit return the most significant bit of z, ignoring the bit or the sign
func (z *Int) MostSignificantBit() uint8 {
	return uint8(z.abs.BitLen() - 1)
}

Almost all what has been added (tests included) were taken/inspired by the uint256 package.