export pset::error::PsetHash

This type appears in the public API as part of the pset::Error type.
However, the type itself is not exported, preventing users from naming
or directly constructing it. This prevents construction of a single
variant of pset::Error, and in particular prevents us from semver-tricking
the Error type.

It is bad practice to export the pset::Error type without also exporting
its constituent types (except as an intentional form of sealing, which
this is not). That is enough reason to accept this PR. But to give a more
elaborate reason:

1. I would like to semver-trick 0.25.3 by re-exporting all the types from
   0.26.0 except the ones that changed (`PartiallySignedTransaction`,
  `TxOut`, `pset::raw::ProprietaryKey`, and types that expose these in
  their public APIs: `Transaction`, `Block` and the sighash cache).

2. I need to re-export the `encode::Error` type, which fortunately did not
   change since 0.25.x, so that I can re-use the Encodable/Decodable logic
   for all the types that I'm re-exporting. (I can either re-export those
   traits directly, along with the error type, or implement a new one with
   impls that pass through to the 0.26.x one ... or retain a *ton* of
   explicit unreviewable code which probably won't even compile without
   changes to handle some types becoming foreign).

3. However, `encode::Error` has conversions to and from `pset::Error`. So
   if I re-export `encode::Error` I need to re-export `pset::Error`. (Also,
   I just really want to; everything I *don't* re-export is a bunch of
   extra explicit code that needs to stick around in this version of the
   library. See above.)

4. But I need to explicitly construct `pset::Error` in the `Map::insert_pair`
   impl for `pset::Output`. (This *can't* be re-exported because it's private,
   and I can't re-export the whole `pset::Output` type because it has a method
   which returns `TxOut`, whose API changed in 0.26.0.)

5. ...so I need a re-export of `PsetHash` to do the construction.

Author: apoelstra

src/pset/error.rs

@@ -26,9 +26,13 @@ use secp256k1_zkp;
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 /// Enum for marking pset hash error
 pub enum PsetHash {
+    /// Bad preimage for `RIPEMD160` hash.
     Ripemd,
+    /// Bad preimage for `SHA256` hash.
     Sha256,
+    /// Bad preimage for `RIPEMD160-SHA256` hash.
     Hash160,
+    /// Bad preimage for double-`SHA256` hash.
     Hash256,
 }
 /// Ways that a Partially Signed Transaction might fail.

src/pset/mod.rs

@@ -53,7 +53,7 @@ use crate::{
 use secp256k1_zkp::rand::{CryptoRng, RngCore};
 use secp256k1_zkp::{self, RangeProof, SecretKey, SurjectionProof};
 
-pub use self::error::{Error, PsetBlindError};
+pub use self::error::{Error, PsetBlindError, PsetHash};
 use self::map::Map;
 pub use self::map::{Global, GlobalTxData, Input, Output, PsbtSighashType, TapTree};