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

examples/signed_pod.rs

@@ -0,0 +1,44 @@
+//! Simple example of building a signed pod and verifying it
+//!
+//! Run: `cargo run --release --example signed_pod`
+use std::collections::HashSet;
+
+use pod2::{
+    backends::plonky2::{primitives::ec::schnorr::SecretKey, signedpod::Signer},
+    frontend::SignedPodBuilder,
+    middleware::{containers::Set, Params, Value},
+};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let params = Params::default();
+
+    // Create a schnorr key pair to sign the pod
+    let sk = SecretKey::new_rand();
+    let pk = sk.public_key();
+    println!("Public key: {:?}\n", pk);
+
+    let mut signer = Signer(sk);
+
+    // Build the signed pod
+    let mut builder = SignedPodBuilder::new(&params);
+    // The values can be String, i64, bool, Array, Set, Dictionary, ...
+    builder.insert("name", "Alice");
+    builder.insert("lucky_number", 42);
+    builder.insert("human", true);
+    let friends_set: HashSet<Value> = ["Bob", "Charlie", "Dave"]
+        .into_iter()
+        .map(Value::from)
+        .collect();
+    builder.insert(
+        "friends",
+        Set::new(params.max_merkle_proofs_containers, friends_set)?,
+    );
+
+    // Sign the pod and verify it
+    let pod = builder.sign(&mut signer)?;
+    pod.verify()?;
+
+    println!("{}", pod);
+
+    Ok(())
+}