Merge #225: Make AssetId::from_inner a const function

1f3b402 Make AssetId::from_inner a const function (Pavel Kokolemin)

Pull request description:

  This will allow constructing `AssetId` values like this:
  ```
  const TESTNET_ASSET: elemets::AssetId = const_asset_id("144c654344aa716d6f3abcc1ca90e5641e4e2a7f633bc09fe3baf64585819a49");

  /// Panics if the asset id string has wrong length or format
  pub const fn const_asset_id(s: &str) -> elements::AssetId {
      let mut data: [u8; 32] = hex_literal::decode::<32>(&[s.as_bytes()]);

      let mut left = 0;
      let mut right = data.len() - 1;
      while left < right {
          let tmp = data[left];
          data[left] = data[right];
          data[right] = tmp;
          left += 1;
          right -= 1;
      }

      elements::AssetId::from_inner(bitcoin::hashes::sha256::Midstate(data))
  }
  ```

  It's a bit of a hack because `hex_literal::decode` should not be used directly.

ACKs for top commit:
  apoelstra:
    ACK 1f3b402; successfully ran local tests; nice, good call

Tree-SHA512: 33e1353ecd0c74df2915509fe24cbd56b5fe9c7b7ce412e4adfa59245c4e4f904e514785ffa0c345b9c3bdffce4c76cb3f9d4d3ce4c9372a80f4b4ce1e9dfa48

Author: apoelstra

src/issuance.rs

@@ -76,7 +76,7 @@ impl AssetId {
     ]));
 
     /// Create an [AssetId] from its inner type.
-    pub fn from_inner(midstate: sha256::Midstate) -> AssetId {
+    pub const fn from_inner(midstate: sha256::Midstate) -> AssetId {
         AssetId(midstate)
     }