This enables audit.Hash to hash time.Time values that may exist as
direct fields in the map. This will error (instead of panic) for any
time.Time values that don't occur within map values. For example, this
does not support a time.Time within a slice. If that needs to be
supported then modifications will need to be made.
This also requires an update to reflectwalk (included in this PR). This
is a minimal change that allows SkipEntry to signal to skip an entire
struct. We do this because we don't want to walk any of time.Time since
we handle it directly.
