Add default hash digest length to table.csv

[nichtich]

Multihash Identifier Registry (derived from table.csv) should include a "default length" value with the number of bytes each has algorithm produces. This is required to check whether a digest length makes sense with a given algorithm (#16). It may also support internal abbreviation of multihashes by omission of the digest length (this is practice anyway as when applications internally stick to a fixed digest length by selection of their data types).