Decode context.Canceled, context.DeadlineExceeded etc as a reference to the stdlib instance

[knz]

Currently, instances of the stdlib errors trasnferred over the network are decoded into an object of the same type/value, but not the same instance. So reference comparisons with == with the stdlib instance fail.

This matters for e.g. context.Context.Err() which describes in its API doc that it returns the standrd errors unwrapped so it's legitimate for a caller to use ==.

We could address that by adding a special case in the leaf decoder, so that if a leaf is one of the known stdlib errors, we return the stdlib instance instead.

[knz]

Some progress made here: #89