More robust and precise error handling.

[bitonic]

When decoding, decoding errors are now isolated from IoErrors.

Decoding is also more strict in various circumstances, see new test.

[bitonic]

(It currently doesn't build because rust-lang-deprecated/rustc-serialize#19 isn't merged)

[TyOverby]

I think this is the way to go, but Ok(try!(foo)) could be replaced with foo.map_err(DecodingError::IoError) and I think it reads clearer. If you wanted to write a to_io function so you could write fo.map_err(to_io), that would probably win any brevity contest.

Also, could you rebase? Github isn't letting me merge.

[bitonic]

Rebased. I see nothing wrong with using try! that way -- that FromError trait seems to serve that purpose well. If you want to change the style using some other function, feel free to do it and merge.

[TyOverby]

Ok, I'll do it post-merge.