Add support for base64url with no padding

[dcoutts]

RFC4648 Section 3.2:

Padding of Encoded Data

In some circumstances, the use of padding ("=") in base-encoded data
is not required or used. In the general case, when assumptions about
the size of transported data cannot be made, padding is required to
yield correct decoded data.

Implementations MUST include appropriate pad characters at the end of
encoded data unless the specification referring to this document
explicitly states otherwise.

As an example of such a specification is RFC7049 (section 2.4.4.2) which has this little snippet:

These three tag types suggest conversions to three of the base data
encodings defined in [RFC4648]. For base64url encoding, padding is
not used (see Section 3.2 of RFC 4648); that is, all trailing equals
signs ("=") are removed from the base64url-encoded string.

So to support such specifications it would be convenient for the Base64.URL modules to provide variants of encode and decode that produce and expect no padding. It's actually especially useful to have the decoder variant since working around the lack of direct support on the encode side is easy, but adding back the correct amount of padding is more involved and expensive.

[andrewthad]

This would also be a useful feature for my purposes as well.

[cartazio]

https://github.com/EButlerIV/u2f/blob/master/src/U2F.hs#L180-L183 might be related?

[23Skidoo]

PRs welcome.

[23Skidoo]

Fixed by #26.