(base64_decode_update): Use base64_decode_single.

Rev: src/nettle/base64-decode.c:1.2
Rev: src/nettle/base64.h:1.10

and streamed operation.

* base64-decode.c: New file.

Rev: src/nettle/base64-decode.c:1.1
Rev: src/nettle/base64-encode.c:1.1