Add deserialize_exact APIs to reject trailing bytes

[tanmay4l]

Fixes #199

[kskalski]

I think this is the correct API we want to have.

Additional thoughts on those modules - I'm looking into cleaning up the serde functions, so far the idea is to:

• remove traits and rely only on free functions (this addition meets the criteria of target API to stay) or let user use Schema* directly (any more complex use-case not permitted by free function)
• switch input slices to impl Reader - current deserialize_exact goes against this direction - alternative would be to enrich Reader with has_data API, which might be a way to go, it could be inefficient for non-slice readers, but given that checking for trailing bytes will only be done on demand with the top level functions, maybe it's fine @cpubot ?
• remove deserialize_from (superceded by deserialize taking Reader)

[cpubot]

I think this API shape is fine as is. We need not consider further API reorganization to ship this functionality. Once we have an agreed upon direction as far as an API re-design / reorganization, we can figure out how to handle in that context.

Another note: we should add clear documentation to these functions as well with doctests.