Simplify decoding (no functional changes)

[ribasushi]

Hi! It looks like the original implementation back in trezor/trezor-crypto@89a7d7797b806fac left a few too many sanity checks in the source. Adjusted and simplified to appease the go compiler.

Thank you for the excellent starting base, and the extensive test suite!

• Before

base58$ go test -bench=. -count=1 -benchmem ./
goos: darwin
goarch: amd64
pkg: github.com/mr-tron/base58
BenchmarkTrivialBase58Encoding-8   	  182612	      7054 ns/op	    1911 B/op	      90 allocs/op
BenchmarkFastBase58Encoding-8      	  928771	      1245 ns/op	     144 B/op	       2 allocs/op
BenchmarkTrivialBase58Decoding-8   	  359553	      3370 ns/op	     513 B/op	      48 allocs/op
BenchmarkFastBase58Decoding-8      	 1558393	       838 ns/op	     368 B/op	       3 allocs/op
PASS
ok  	github.com/mr-tron/base58	58.666s

• After ( when including Simplify encoding (no functional changes) #15 )

base58$ go test -bench=. -count=1 -benchmem ./
goos: darwin
goarch: amd64
pkg: github.com/mr-tron/base58
BenchmarkTrivialBase58Encoding-8   	  155588	      7466 ns/op	    1911 B/op	      90 allocs/op
BenchmarkFastBase58Encoding-8      	  966726	      1245 ns/op	      96 B/op	       2 allocs/op
BenchmarkTrivialBase58Decoding-8   	  350648	      3447 ns/op	     513 B/op	      48 allocs/op
BenchmarkFastBase58Decoding-8      	 2276010	       523 ns/op	     127 B/op	       2 allocs/op
PASS
ok  	github.com/mr-tron/base58	57.326s

/cc @prusnak @Stebalien

[mr-tron]

Thank you. I don't have time today, but I will check and merge your pull requests tomorrow.

[ribasushi]

I don't have time today, but I will check and merge your pull requests tomorrow.

@mr-tron no worries, I clearly wasn't done anyway :) I think now I am done with tweaking all of this, the 3 PRs are good to look at, whenever time permits.

Thanks!

[ribasushi]

Hi there @mr-tron! Do you think you will have a chance to look at these before next Monday? Thanks!

[mr-tron]

I merged. You can also add your name into LICENSE file if you want.