Merge #176: fix: vbavailable map in GetBlockTemplateResult

1f5c57a fix: version_bits_available maps String to u32 (0xb10c)

Pull request description:

  PR #163 implemented support for `getblocktemplate`. However, during the implementation the `version_bits_available` HashMap the map fields were switched.

  This became apparent during testing against a v0.21.1 Bitcoin Core node, which includes support for the Taproot softfork via the 2nd version bit. Previously, no softfork was defined.

  ```
  $ bitcoin-cli getblocktemplate "{\"rules\": [\"segwit\"]}"
  ...
    "vbavailable": {
      "taproot": 2
    },
  ...
  ```

ACKs for top commit:
  sgeisler:
    utACK 1f5c57a
  tcharding:
    utACK 1f5c57a

Tree-SHA512: ebae449b93ca8d428e8c0e6dcbeca20765966500e3582368b1a6dc3a60e3205c507121b06a22b3e0e4091efc4828cc17dd65049afebf09695d9a5abc50b1dc5c

Author: apoelstra

json/src/lib.rs

@@ -1513,7 +1513,7 @@ pub struct GetBlockTemplateResult {
     pub capabilities: Vec<GetBlockTemplateResultCapabilities>,
     /// Set of pending, supported versionbit (BIP 9) softfork deployments
     #[serde(rename = "vbavailable")]
-    pub version_bits_available: HashMap<u32, String>,
+    pub version_bits_available: HashMap<String, u32>,
     /// Bit mask of versionbits the server requires set in submissions
     #[serde(rename = "vbrequired")]
     pub version_bits_required: u32,