`Type.size_of` currently behaves oddly in the case where it is passed: - a representation of a fixed-length type; and - a value that does not have that length. We should decide on a consistent strategy for dealing with these cases. See discussion in https://github.com/mirage/irmin/pull/1030#discussion_r453362110. _(Originally reported at https://github.com/mirage/irmin/issues/1037.)_