Edward Z. Yang authored Nov 02, 2016 and Edward Z. Yang committed Nov 05, 2016

The idea is we can use Rep to get a full, structural representation
of a type, and the fingerprint it using Typeable.  This gives
us a very concise way of fingerprinting our Binary representation.

This patch is not completely correct; the fingerprint needs
to be overridable when someone writes a custom Binary instance.
But this should be "good enough" in practice; we're not using
these fingerprints to check anything security critical.

TODO: Not sure if I have tagged all the call-sites which could
profit from this.

Fixes #4059.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>