Add some top-level examples (#303)

The examples show:
- Building a Signed Pod with different types of values
- Building a MainPod
- Input SignedPod to MainPod
- Input MainPod to MainPod
- Using MainPod or MockMainPod
- Using custom predicates

src/frontend/operation.rs

@@ -45,9 +45,9 @@ impl fmt::Display for OperationArg {
     }
 }
 
-impl From<Value> for OperationArg {
-    fn from(v: Value) -> Self {
-        Self::Literal(v)
+impl<V: Into<Value>> From<V> for OperationArg {
+    fn from(value: V) -> Self {
+        Self::Literal(value.into())
     }
 }
 
@@ -57,24 +57,6 @@ impl From<&Value> for OperationArg {
     }
 }
 
-impl From<&str> for OperationArg {
-    fn from(s: &str) -> Self {
-        Self::Literal(Value::from(s))
-    }
-}
-
-impl From<i64> for OperationArg {
-    fn from(v: i64) -> Self {
-        Self::Literal(Value::from(v))
-    }
-}
-
-impl From<bool> for OperationArg {
-    fn from(b: bool) -> Self {
-        Self::Literal(Value::from(b))
-    }
-}
-
 impl From<(&SignedPod, &str)> for OperationArg {
     fn from((pod, key): (&SignedPod, &str)) -> Self {
         // TODO: TryFrom.