Use VLQ varints in payload (casey#24)

src/directive.rs

@@ -0,0 +1,8 @@
+use super::*;
+
+#[derive(Default, Serialize, Debug, PartialEq)]
+pub struct Directive {
+  pub id: u128,
+  pub amount: u128,
+  pub output: u128,
+}

src/error.rs

@@ -0,0 +1,26 @@
+use super::*;
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+  Script(script::Error),
+  Opcode(opcodes::All),
+  Varint,
+}
+
+impl From<script::Error> for Error {
+  fn from(error: script::Error) -> Self {
+    Self::Script(error)
+  }
+}
+
+impl Display for Error {
+  fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+    match self {
+      Self::Script(err) => write!(f, "failed to parse script: {err}"),
+      Self::Opcode(op) => write!(f, "non-push opcode {op} in payload"),
+      Self::Varint => write!(f, "varint over maximum value"),
+    }
+  }
+}
+
+impl std::error::Error for Error {}