derive: add XpubDerivable constructors

Closes #27

derive/src/path.rs

@@ -43,6 +43,24 @@ pub enum DerivationParseError {
 #[derive(Clone, Eq, PartialEq, Hash, Debug)]
 pub struct DerivationSeg<I: IdxBase = NormalIndex>(Confined<BTreeSet<I>, 1, 8>);
 
+impl<I: IdxBase> From<&'static [I]> for DerivationSeg<I> {
+    fn from(indexes: &'static [I]) -> Self {
+        Self(Confined::from_iter_unsafe(indexes.into_iter().copied()))
+    }
+}
+
+impl<I: IdxBase> From<[I; 2]> for DerivationSeg<I> {
+    fn from(indexes: [I; 2]) -> Self { Self(Confined::from_iter_unsafe(indexes)) }
+}
+
+impl<I: IdxBase> From<[I; 3]> for DerivationSeg<I> {
+    fn from(indexes: [I; 3]) -> Self { Self(Confined::from_iter_unsafe(indexes)) }
+}
+
+impl<I: IdxBase> From<[I; 4]> for DerivationSeg<I> {
+    fn from(indexes: [I; 4]) -> Self { Self(Confined::from_iter_unsafe(indexes)) }
+}
+
 impl<I: IdxBase> DerivationSeg<I> {
     pub fn new(index: I) -> Self { DerivationSeg(confined_bset![index]) }
 

derive/src/xpub.rs

@@ -495,7 +495,33 @@ pub struct XpubDerivable {
     pub(crate) keychains: DerivationSeg<Keychain>,
 }
 
+impl From<XpubSpec> for XpubDerivable {
+    fn from(spec: XpubSpec) -> Self {
+        XpubDerivable {
+            spec,
+            variant: None,
+            keychains: DerivationSeg::from([Keychain::INNER, Keychain::OUTER]),
+        }
+    }
+}
+
 impl XpubDerivable {
+    pub fn new(xpub: Xpub, origin: XpubOrigin) -> Self {
+        XpubDerivable {
+            spec: XpubSpec::new(xpub, origin),
+            variant: None,
+            keychains: DerivationSeg::from([Keychain::INNER, Keychain::OUTER]),
+        }
+    }
+
+    pub fn with(xpub: Xpub, origin: XpubOrigin, keychains: &'static [Keychain]) -> Self {
+        XpubDerivable {
+            spec: XpubSpec::new(xpub, origin),
+            variant: None,
+            keychains: DerivationSeg::from(keychains),
+        }
+    }
+
     pub fn xpub(&self) -> Xpub { self.spec.xpub }
 
     pub fn origin(&self) -> &XpubOrigin { &self.spec.origin }