Fast Implementation of Base58 encoding

Fast implementation of base58 encoding in Go.

Base algorithm is copied from https://github.com/trezor/trezor-crypto/blob/master/base58.c

Benchmark

Trivial - encoding via big.Int (over libraries use this implemenation) Fast - optimized algorythm from trezor

BenchmarkTrivialBase58Encoding-4   	  200000	     10602 ns/op  
BenchmarkFastBase58Encoding-4      	 1000000	      1637 ns/op
BenchmarkTrivialBase58Decoding-4   	  200000	      8316 ns/op
BenchmarkFastBase58Decoding-4      	 1000000	      1045 ns/op

Encoding - faster by 6 times

Decoding - faster by 8 times

Usage example

package main

import (
	"fmt"
	"github.com/mr-tron/base58"
)

func main() {

	encoded := "1QCaxc8hutpdZ62iKZsn1TCG3nh7uPZojq"
	num, err := base58.Decode(encoded)
	if err != nil {
		fmt.Printf("Demo %v, got error %s\n", encoded, err)	
	}
	chk := base58.Encode(num)
	if encoded == string(chk) {
		fmt.Printf ( "Successfully decoded then re-encoded %s\n", encoded )
	} 
}