generic stubbing/expansion support

[maxtaco]

• you can stub out any part of the inner json by replacing it with a hash of the sub-object, and then you can pass the expansions separately
• support for both generating and verifying these objects
• some tests
• for example, before:

{
  "body": {
    "cryptocurrency": {
      "address": "1BjgMvwVkpmmJ5HFGZ3L3H1G6fcKLNGT5h",
      "type": "bitcoin"
    },
    "key": {
      "host": "keybase.io",
      "kid": "012085b48c269ab5b17e07a64fbbc378ca4201dcc26bff4c1ba3fcb99eeaf0b6b69a0a",
      "uid": "389b29bec041e89fe4fd3c9d50a15619",
      "username": "u_f1977962b6"
    },
    "type": "cryptocurrency",
    "version": 2
  },
  "ctime": 1578670773,
  "expire_in": 157680000,
  "prev": null,
  "seq_type": 1,
  "seqno": 1,
  "tag": "signature"
}

• and after:

  
  {
  "body": {
  "cryptocurrency": "ccba4b68dc528896a423ccda1320ef9f84e15ae6345cf0140803b10f77841654",
  "key": {
    "host": "keybase.io",
    "kid": "012085b48c269ab5b17e07a64fbbc378ca4201dcc26bff4c1ba3fcb99eeaf0b6b69a0a",
    "uid": "389b29bec041e89fe4fd3c9d50a15619",
    "username": "u_f1977962b6"
  },
  "type": "cryptocurrency",
  "version": 2
  },
  "ctime": 1578670773,
  "expire_in": 157680000,
  "prev": null,
  "seq_type": 1,
  "seqno": 1,
  "tag": "signature"
  }

- with the following "expansions" dictionary:

```json
{
  "ccba4b68dc528896a423ccda1320ef9f84e15ae6345cf0140803b10f77841654": {
    "address": "1BjgMvwVkpmmJ5HFGZ3L3H1G6fcKLNGT5h",
    "type": "bitcoin",
    "entropy": "O45K0tpXSft1nB0FqPoDmw=="
  }
}

[maxtaco]

We're doing this so that we can delete, selectively, certain parts of the sigchain by withholding the expansions.

[maxtaco]

Note in the above example, that before any item is stubbed out, entropy is added in, so that someone trying to reconstruct the deleted portion can't guess.