Use base64.RawURLEncoding to avoid padding

JWT uses a non-padded base64 encoding.

Current code uses base64.URLEncoding to generate a padded string and
then removes the padding.
Likewise, current code adds padding before decoding.

Instead, use base64.RawURLEncoding which does not add or require the
padding in the first place.

In addition to making the code cleaner, this reduces memory allocations
as reported by benchmarks.

Before:
```
BenchmarkECDSASigning/Basic_ES256-8                     191396         6917 ns/op       4121 B/op         61 allocs/op
BenchmarkECDSASigning/Basic_ES256/sign-only-8            49347        25039 ns/op       3201 B/op         39 allocs/op
BenchmarkECDSASigning/basic_ES256_invalid:_foo_=>_bar-8 190668         6586 ns/op       4121 B/op         61 allocs/op
BenchmarkHS256Signing-8                                1260060         1131 ns/op       1585 B/op         32 allocs/op
BenchmarkHS384Signing-8                                 861378         1387 ns/op       1969 B/op         32 allocs/op
BenchmarkHS512Signing-8                                 896745         1463 ns/op       2065 B/op         32 allocs/op
BenchmarkRS256Signing-8                                   3086       355769 ns/op      32576 B/op        136 allocs/op
BenchmarkRS384Signing-8                                   3414       353570 ns/op      32694 B/op        136 allocs/op
BenchmarkRS512Signing-8                                   3235       349394 ns/op      32706 B/op        136 allocs/op
```

After:
```
BenchmarkECDSASigning/Basic_ES256-8                     176617         6827 ns/op       4021 B/op         58 allocs/op
BenchmarkECDSASigning/Basic_ES256/sign-only-8            48038        24213 ns/op       3169 B/op         38 allocs/op
BenchmarkECDSASigning/basic_ES256_invalid:_foo_=>_bar-8 194352         6928 ns/op       4021 B/op         58 allocs/op
BenchmarkHS256Signing-8                                1000000         1127 ns/op       1488 B/op         29 allocs/op
BenchmarkHS384Signing-8                                 972552         1369 ns/op       1873 B/op         29 allocs/op
BenchmarkHS512Signing-8                                 780751         1368 ns/op       1969 B/op         29 allocs/op
BenchmarkRS256Signing-8                                   3014       387326 ns/op      32475 B/op        133 allocs/op
BenchmarkRS384Signing-8                                   3044       361411 ns/op      32591 B/op        133 allocs/op
BenchmarkRS512Signing-8                                   3273       355504 ns/op      32607 B/op        133 allocs/op
```

Benchmarks of signing methods ES384 and ES512 are omitted because their
allocations are not consistent.

Author: jkanywhere

token.go

@@ -95,14 +95,10 @@ func ParseWithClaims(tokenString string, claims Claims, keyFunc Keyfunc) (*Token
 
 // Encode JWT specific base64url encoding with padding stripped
 func EncodeSegment(seg []byte) string {
-	return strings.TrimRight(base64.URLEncoding.EncodeToString(seg), "=")
+	return base64.RawURLEncoding.EncodeToString(seg)
 }
 
 // Decode JWT specific base64url encoding with padding stripped
 func DecodeSegment(seg string) ([]byte, error) {
-	if l := len(seg) % 4; l > 0 {
-		seg += strings.Repeat("=", 4-l)
-	}
-
-	return base64.URLEncoding.DecodeString(seg)
+	return base64.RawURLEncoding.DecodeString(seg)
 }