Update default hamt hash function to sha256 and make algo generic

[austinabell]

Summary of changes
Changes introduced in this pull request:

• ref: https://github.com/ipfs/go-hamt-ipld/pull/49
  • Done for security reasons (hash size bumped to 32 bytes) but also made generic to allow swapping this hash function more easily (and I didn't necessarily like the feature gating that I did before).
• Fixed bugs in using wrong bit length or type (key types matter as they get serialized, even if they hash the same)

The defaults for the generic types required that I swap the order of the generics, which unfortunately required to touch a bunch of files, but is necessary as all defaulted must be the last parameters. And if you see Hamt::<_>::.. it's just because you need to explicitly default the types, and can't leave out.

Reference issue to close (if applicable)

Closes #614

Other information and links

[ec2]

LGTM