Model {Value,Asset,Nonce}::Confidential using secp256k1-zkp types

This patch introduces a _breaking_ change in the (de)serialisation of
confidential assets, confidential values and confidential nonces. In
particular, we now support both readable and compact serialization,
and they both include a serialization prefix `2` as opposed to the old
prefix which is now part of the inner condifential value, since that
is controlled by secp256k1-zkp. We choose the number `2` since we were
already using `0` for null values and `1` for explicit values.

We also had to modify some of the test data now that `secp256k1-zkp` is
validating for us that the confidential assets are actually valid
generators.

Cargo.toml

@@ -14,12 +14,14 @@ default = [ "json-contract" ]
 json-contract = [ "serde_json" ]
 "serde-feature" = [
     "bitcoin/use-serde",
+    "secp256k1-zkp/use-serde",
     "serde"
 ]
 "fuzztarget" = []
 
 [dependencies]
 bitcoin = "0.26"
+secp256k1-zkp = { version = "0.2", features = [ "global-context", "hashes" ] }
 slip21 = "0.2.0"
 
 # While this dependency is included in bitcoin, we need this to use the macros.
@@ -37,3 +39,7 @@ rand = "0.6.5"
 serde_test = "1.0"
 serde_json = "<=1.0.44"
 ryu = "<1.0.5"
+
+[patch.crates-io]
+# Until https://github.com/ElementsProject/rust-elements/pull/70/ is merged
+secp256k1-zkp = { git = "https://github.com/luckysori/rust-secp256k1-zkp", branch = "fix-pedersen-deser" }