Two encoding-related cleanups (#82)

* Remove superfluous bitmasking in `encode`

We only need to mask with `&0x1F` once per generated character, rather
than twice as we did in many cases. As long as the `&0x1F` operation is
last, we will always throw away the top three bits as expected.

* Remove use of `unsafe` in `String()` method

Now that an `Encode()` method was added in #56, it is less necessary to
resort to unsafe methods in `String()`. This also appears to have
minimal benefit, as seen by these before (using unsafe) and after
(without unsafe) benchmarks:

Before (using unsafe):

% go test -benchmem -bench .
goos: darwin
goarch: arm64
pkg: github.com/rs/xid
BenchmarkNew-8          	21845306	        52.34 ns/op	       0 B/op	       0 allocs/op
BenchmarkNewString-8    	22674129	        51.18 ns/op	       0 B/op	       0 allocs/op
BenchmarkFromString-8   	286217863	         4.233 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/rs/xid	4.142s

After (without unsafe):

% go test -benchmem -bench .
goos: darwin
goarch: arm64
pkg: github.com/rs/xid
BenchmarkNew-8          	21423278	        53.01 ns/op	       0 B/op	       0 allocs/op
BenchmarkNewString-8    	21977233	        53.67 ns/op	       0 B/op	       0 allocs/op
BenchmarkFromString-8   	289641164	         4.162 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/rs/xid	4.181s

id.go

@@ -54,7 +54,6 @@ import (
 	"sort"
 	"sync/atomic"
 	"time"
-	"unsafe"
 )
 
 // Code inspired from mgo/bson ObjectId
@@ -172,7 +171,7 @@ func FromString(id string) (ID, error) {
 func (id ID) String() string {
 	text := make([]byte, encodedLen)
 	encode(text, id[:])
-	return *(*string)(unsafe.Pointer(&text))
+	return string(text)
 }
 
 // Encode encodes the id using base32 encoding, writing 20 bytes to dst and return it.
@@ -206,23 +205,23 @@ func encode(dst, id []byte) {
 
 	dst[19] = encoding[(id[11]<<4)&0x1F]
 	dst[18] = encoding[(id[11]>>1)&0x1F]
-	dst[17] = encoding[(id[11]>>6)&0x1F|(id[10]<<2)&0x1F]
+	dst[17] = encoding[(id[11]>>6)|(id[10]<<2)&0x1F]
 	dst[16] = encoding[id[10]>>3]
 	dst[15] = encoding[id[9]&0x1F]
 	dst[14] = encoding[(id[9]>>5)|(id[8]<<3)&0x1F]
 	dst[13] = encoding[(id[8]>>2)&0x1F]
 	dst[12] = encoding[id[8]>>7|(id[7]<<1)&0x1F]
-	dst[11] = encoding[(id[7]>>4)&0x1F|(id[6]<<4)&0x1F]
+	dst[11] = encoding[(id[7]>>4)|(id[6]<<4)&0x1F]
 	dst[10] = encoding[(id[6]>>1)&0x1F]
-	dst[9] = encoding[(id[6]>>6)&0x1F|(id[5]<<2)&0x1F]
+	dst[9] = encoding[(id[6]>>6)|(id[5]<<2)&0x1F]
 	dst[8] = encoding[id[5]>>3]
 	dst[7] = encoding[id[4]&0x1F]
 	dst[6] = encoding[id[4]>>5|(id[3]<<3)&0x1F]
 	dst[5] = encoding[(id[3]>>2)&0x1F]
 	dst[4] = encoding[id[3]>>7|(id[2]<<1)&0x1F]
-	dst[3] = encoding[(id[2]>>4)&0x1F|(id[1]<<4)&0x1F]
+	dst[3] = encoding[(id[2]>>4)|(id[1]<<4)&0x1F]
 	dst[2] = encoding[(id[1]>>1)&0x1F]
-	dst[1] = encoding[(id[1]>>6)&0x1F|(id[0]<<2)&0x1F]
+	dst[1] = encoding[(id[1]>>6)|(id[0]<<2)&0x1F]
 	dst[0] = encoding[id[0]>>3]
 }