add Address::is_liquid

User code can already compare against AddressParams, but it seems more
easy to find a method to check if the address is liquid.
Even thoug is less general because you can't discriminate between
elements and liquid testnet, most of the time wallets are interested
only in the mainnet or not discrimination.

Author: RCasatta

src/address.rs

@@ -223,6 +223,11 @@ impl Address {
         self.blinding_pubkey.is_some()
     }
 
+    /// Return if the address is for the Liquid network
+    pub fn is_liquid(&self) -> bool {
+        self.params == &AddressParams::LIQUID
+    }
+
     /// Creates a pay to (compressed) public key hash address from a public key
     /// This is the preferred non-witness type address
     #[inline]