Support serde w/o std and update base64 dependency

[mina86]

By using base64’s encode_slice and decode_slice it’s possible to
implement serde serialisation without std or alloc. Take advantage of
that and allow enabling serde support separate from std. This is
gated by a new serde feature which.

To maintain backwards-compatibility make std feature enables serde
feature. Ideally, it should be possible to enable std without
bringing in serde support. We're still in 0.x releases so there’s
technically no semver issue in breaking builds, but for now it’s
probably nicer to keep std enabling serde support. It probably
makes sense to remove that in 0.8.

With all that, make serde and base64 dependencies optional.
Previously, they were always included even though they were only used
if std feature was enabled. Now, they are enabled only if
explicitly requested.

While at it, update base64 dependency to the newest 0.21.2 release.

[mina86]

@sorpaas, @alvicsam, friendly ping