Validate in struct and union constructors

[abhinav]

This makes validation happen in the constructors for struct and union instead of doing it during serialization. The intention is to fail as early as possible.

Also, numeric types are more liberals about what values count as valid. Most integral values can be casted into an i64, for example. We still don't do bounds checks, though.

Finally, BinaryTypeSpec was changed to be similar to TextTypeSpec. It now accepts unicode and automatically converts it into bytes.

Resolves #31. Resolves #33.

[abhinav]

@blampe @breerly @junchaowu

[blampe]

Looks great.