Safer serializations

[GarrickOllivander]

Safer serializations.
Removed xx:from_vec constructors as they did not check length or paniced.

[ignopeverell]

Going in the right direction, you're right about removing the from_vec constructors. I just don't like adding all those specialized readers and dependencies in ser.rs and the related base traits. How about doing Proof::read or Hash::read instead of read_proof and read_hash?

[GarrickOllivander]

review please. Can't move Commitment and RangeProof out of ser.rs as doing so would introduce cyclical dependency between core and secp.

[ignopeverell]

As there were a lot of small additive changes I ended up squashing the merge to keep the history cleaner. Hope you don't mind.

[GarrickOllivander]

That was the right thing to do, thank you.

[ignopeverell]

Thanks for another great pull request!