Add unstable constructor to Witness

In preparation for moving the `Witness` to `primitives` we need a way to
construct the `Witness` when decoding. In order to maintain the current
performance and not introduce additional allocations we need to be able
to construct a `Witness` from the content buffer, this leaks the
implementation details of `Witness`.

Add a clearly marked unstable constructor to create a `Witness` from
parts. This introduces the concept of `foo__unstable` function names;
add a section to the README describing unstable functions and semver
guarantees.

README.md

@@ -52,6 +52,13 @@ are no plans to do so. Of course, patches to fix specific consensus incompatibil
 16-bit pointer sizes are not supported and we can't promise they will be. If you care about them
 please let us know, so we can know how large the interest is and possibly decide to support them.
 
+### Semver compliance
+
+We try hard to maintain strict semver compliance with our releases. This codebase includes some
+public functions marked unstable (e.g., `pub fn foo__unstable()`). These functions do not adhere to
+semver rules; use them at your own discretion.
+
+
 ## Documentation
 
 Currently can be found on [docs.rs/bitcoin](https://docs.rs/bitcoin/). Patches to add usage examples

bitcoin/src/blockdata/witness.rs

@@ -193,7 +193,8 @@ impl Decodable for Witness {
             content.truncate(cursor);
             // Index space is now at the end of the Vec
             content.rotate_left(witness_index_space);
-            Ok(Witness { content, witness_elements, indices_start: cursor - witness_index_space })
+            let indices_start = cursor - witness_index_space;
+            Ok(Witness::from_parts__unstable(content, witness_elements, indices_start))
         }
     }
 }
@@ -248,6 +249,19 @@ impl Witness {
         Witness { content: Vec::new(), witness_elements: 0, indices_start: 0 }
     }
 
+    /// Creates a new [`Witness`] from inner parts.
+    ///
+    /// This function leaks implementation details of the `Witness`, as such it is unstable and
+    /// should not be relied upon (it is primarily provided for use in `rust-bitcoin`).
+    ///
+    /// UNSTABLE: This function may change, break, or disappear in any release.
+    #[inline]
+    #[doc(hidden)]
+    #[allow(non_snake_case)]    // Because of `__unstable`.
+    pub fn from_parts__unstable(content: Vec<u8>, witness_elements: usize, indices_start: usize) -> Self {
+        Witness { content, witness_elements, indices_start }
+    }
+
     /// Creates a witness required to spend a P2WPKH output.
     ///
     /// The witness will be made up of the DER encoded signature + sighash_type followed by the