[btcsuite/btcutil] dca623: Optimize base58 decoding.

[Josh Rickmar]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcutil
  Commit: dca623d4ef069c804bf1c11274be8d8419c105a3
      
https://github.com/btcsuite/btcutil/commit/dca623d4ef069c804bf1c11274be8d8419c105a3
  Author: Josh Rickmar <jr...@conformal.com>
  Date:   2015-01-24 (Sat, 24 Jan 2015)

  Changed paths:
    A base58/alphabet.go
    M base58/base58.go
    A base58/genalphabet.go

  Log Message:
  -----------
  Optimize base58 decoding.

This change introduces an autogenerated base58 digit table to remove
the need to find the index of a character in the modified base58
alphabet each time.  Additionally, it removes some unnecessary big
integer allocations to cut down on the GC churn.

Before:
BenchmarkBase58Encode         20          64998995 ns/op           0.08 MB/s
BenchmarkBase58Decode         50          35965928 ns/op           0.19 MB/s

Now:
BenchmarkBase58Encode         20          64644351 ns/op           0.08 MB/s
BenchmarkBase58Decode        200           7914748 ns/op           0.86 MB/s