Define valid equality/hashCode on Schema

[jdegoes]

Schema must have well-defined equality and hashCode methods.

Note that due to the presence of Schema#transform, which stores functions in the ADT, we will have to use reference equality on functions to check for equality and to compute hash code. Reference equality implies equality, but is not necessary for equality, which means that two schemas could in fact be equal, but equals will return false. That's a known and acceptable limitation of this approach.

We should have a test suite that demonstrates equal schemas compare as equal and hash the same.