… less than or equal to modulus highest limb (#120)

* optimize AddMod where modulus is 4-limbs wide and x/y highest limb is lte modulus highest limb

* gofmt

This change adds various string-conversion methods, mainly from decimal. 
It also aligns some setters, into the following: 

Public "convenience" constructors
    FromBig(*big.Int)
    FromHex(string)
    FromBase10(string)
Instance setters:
    (z *Int) SetFromBase10(string) (possibly)
    (z *Int) SetFromBig(*big.Int)
    (z *Int) SetBytes([]byte)
    (z *Int) SetString(string, int)


Co-authored-by: a <[email protected]>
Co-authored-by: Tjudice <[email protected]>
Co-authored-by: Martin Holst Swende <[email protected]>

Remove some complains from deepsource, no functional changes

Adds support for using uint256 in ssz contexts, by implementing the fastssz Marshaler interface. 

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Co-authored-by: Martin Holst Swende <[email protected]>

This change optimizes the Scan method, by making utilizing seek instead of string-split, adds some test and makes the function alloc-free. 

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Co-authored-by: Martin Holst Swende <[email protected]>

Add support for MustFromBig for nicer struct initialization. MustFromBig panics if the conversion overflows.

…ng (#130)

This PR implements the Dec method 'natively', and adds the API-method PrettyDec.

	// Dec returns the decimal representation of z.
	func (z *Int) Dec() string
	
	// PrettyDec returns the decimal representation of z, with thousands-separators.
	func (z *Int) PrettyDec(separator byte) string

It also adds benchmarks and a new fuzz-test for the changed methods.

The PR #128 added MustFromBig, this PR adds two similar helpers: MustFromHex and MustFromDecimal.

	// MustFromHex is a convenience-constructor to create an Int from
	// a hexadecimal string.
	// Returns a new Int and panics if any error occurred.
	func MustFromHex(hex string) *Int
	
	// MustFromDecimal is a convenience-constructor to create an Int from a
	// decimal (base 10) string.
	// Returns a new Int and panics if any error occurred.
	func MustFromDecimal(decimal string) *Int