mx.api 0.2.0

First feature update after the 0.1.0 CRAN release. Currently in CRAN's
incoming queue.

Highlights

• New transport endpoints for end-to-end-encryption coordination:
  • mx_keys_upload() -- publish device keys and one-time keys.
  • mx_keys_query() -- fetch device keys for one or more users.
  • mx_keys_claim() -- claim one-time keys for an Olm handshake.
  • mx_send_to_device() -- send a to-device event (e.g. an
    encrypted Olm payload).
    The package itself remains crypto-free; these carry already-signed
    payloads built by an external signer.
• mx_canonical_json() -- byte-stable Matrix-spec canonical JSON
  encoder, the input format Matrix's signing rules require. 107
  unit-test assertions cover UTF-8 sort, integer-vs-float, NaN/Inf/NA
  rejection, control-char escaping, duplicate-key rejection, and the
  realistic /keys/upload payload shape. Intentionally hand-rolled
  rather than a jsonlite wrapper so the spec-sensitive choices are
  visible and tested.

Pairs with

• mx.crypto for Olm +
  Megolm cryptographic operations on the payloads these endpoints
  carry.