types: Replace expression-based SpendPolicy JSON marshalling

[lukechampine]

The expression-based SpendPolicy encoding is more compact and easier to read, but using it as the standard JSON encoding means that all clients have to write a custom rule, which is annoying. Since most users won't be looking at raw JSON anyway, we might as well make life a bit easier for our devs by switch to a more conventional structural encoding.
(Note, however, that we still intend to use the expression-based encoding when displaying policies in UIs.)

Thanks to @Alrighttt for surfacing this.

Old:

thresh(2, [
  above(100),
  pk(0x111d2995afa8bf162180a647b9f1eb6a275fe8818e836b69b351871d5caf9c590ed25aec0616),
  thresh(2, [
    above(200),
    pk(0x111d2995afa8bf162180a647b9f1eb6a275fe8818e836b69b351871d5caf9c590ed25aec0616),
  ]),
]),

New:

{
  "type": "thresh",
  "policy": {
    "n": 2,
    "of": [
      {
        "type": "above",
        "policy": 100
      },
      {
        "type": "pk",
        "policy": "ed25519:111d2995afa8bf162180a647b9f1eb6a275fe8818e836b69b351871d5caf9c590ed25aec0616"
      },
      {
        "type": "thresh",
        "policy": {
          "n": 2,
          "of": [
            {
              "type": "above",
              "policy": 200
            },
            {
              "type": "pk",
              "policy": "ed25519:111d2995afa8bf162180a647b9f1eb6a275fe8818e836b69b351871d5caf9c590ed25aec0616"
            }
          ]
        }
      }
    ]
  }
}