Change interface to require `hasher` and `codec` to match `multiformats/block` requirements

[inverted-capital]

In this line Block is called from the multiformats pacakge after having renamed blockCodec to codec and blockHasher to hasher.

Having this internal name change means a minor inconvenience if working with the multiformats/block library in the same file as that calling the hamt constructors. Bringing this interface in line seems better, altho admittedly very minor.

Possibly could the interfaces be made to match, as the gain from renaming is unclear ?