A small refactor to move this constant closer to the `enum` that it is
about. An associated constant constant fits more nicely.
Reason for changing the name from `SHA1_*_LEN` to `LEN_SHA1`: As we
think about the object format in the future becoming variable, we
would have that the length of an OID is parameterized by the object
format. So, we imagine a function `fn len(format: ObjectFormat)`, to
be called as `Oid::len(ObjectFormat::Sha1)`. This is the new order
we use.
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