miniscript: remove MalleablePkH script context rule

This rule is weird and wrong in a number of ways. It was introduced in
PR #97 c93bdef which was the massive PR
that introduced the Ctx parameter. This particular rule wasn't commented
on, but we can infer some things:

* From the docs on the error and my vague memory, the problem was that
  we couldn't estimate satisfaction for a pubkeyhash fragment because we
  didn't know if the key was 33 or 65 bytes ahead of time.
* Therefore, the code bans this in a legacy context, but not in bare
  contexts (in bare contexts there is a comment saying that bare
  fragments "can't contain miniscript because of standardness rules" so
  maybe we thought about this ... but that comment has never been true,
  so we thought wrong..). We should've banned the fragment in bare
  contexts as well.
* Also, while the error is called MalleablePkH, none of this has
  anything to do with malleability!

Later, in #431, we introduced the RawPkH fragment and put a pubkey into
the PkH fragment. At that point, for PkH we always knew the pubkey and
could just directly compute its size using the `is_uncompressed`
accessor. The RawPkH fragment was a second-class citizen which didn't
even have a string serialization. At this point we should have dropped
the MalleablePkH rule.

Still later, in #461, we introduced `ExtParams` and the ability to parse
"insane" scripts in a more flexible way, including enabling the
experimental `expr_raw_raw_pkh` fragment. We then gained an error
`Analyzable::ContainsRawPkh` which conceptually overlaps with
`ScriptContextError::MalleablePkH` but which isn't tied to any context,
just whether the user has enabled this second-class feature.

Author: apoelstra

src/miniscript/context.rs

@@ -21,10 +21,6 @@ use crate::{hash256, Error, ForEachKey, Miniscript, MiniscriptKey, Terminal};
 /// Error for Script Context
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub enum ScriptContextError {
-    /// Script Context does not permit PkH for non-malleability
-    /// It is not possible to estimate the pubkey size at the creation
-    /// time because of uncompressed pubkeys
-    MalleablePkH,
     /// Script Context does not permit OrI for non-malleability
     /// Legacy fragments allow non-minimal IF which results in malleability
     MalleableOrI,
@@ -74,8 +70,7 @@ impl error::Error for ScriptContextError {
         use self::ScriptContextError::*;
 
         match self {
-            MalleablePkH
-            | MalleableOrI
+            MalleableOrI
             | MalleableDupIf
             | CompressedOnly(_)
             | XOnlyKeysNotAllowed(_, _)
@@ -97,7 +92,6 @@ impl error::Error for ScriptContextError {
 impl fmt::Display for ScriptContextError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
-            ScriptContextError::MalleablePkH => write!(f, "PkH is malleable under Legacy rules"),
             ScriptContextError::MalleableOrI => write!(f, "OrI is malleable under Legacy rules"),
             ScriptContextError::MalleableDupIf => {
                 write!(f, "DupIf is malleable under Legacy rules")
@@ -380,8 +374,6 @@ impl ScriptContext for Legacy {
         frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
         match *frag {
-            Terminal::PkH(ref _pkh) => Err(ScriptContextError::MalleablePkH),
-            Terminal::RawPkH(ref _pk) => Err(ScriptContextError::MalleablePkH),
             Terminal::OrI(ref _a, ref _b) => Err(ScriptContextError::MalleableOrI),
             Terminal::DupIf(ref _ms) => Err(ScriptContextError::MalleableDupIf),
             _ => Ok(()),