feat: implement comparison skeleton

This commit lays out the general idea of how the ComparisonData struct will work. It is expandable and I'm sure we will add more to it as time goes on.

src/common/comparison/utils.rs

@@ -0,0 +1,28 @@
+use crate::common::ValueType;
+
+/// This function is a work in progress. It's designed to flatten a list of maps by using the
+/// given key and verifier function, however it currently doesn't work and just needs to be
+/// planned out a little more.
+fn flatten_map_helper<F>(
+    list: &Vec<ValueType>,
+    key: &String,
+    verifier: F,
+) -> Result<Vec<ValueType>, String>
+where
+    F: Fn(&ValueType) -> Option<&ValueType>,
+{
+    let mut out = Vec::new();
+    for item in list {
+        let map = match item {
+            ValueType::Map(Some(map)) => map,
+            _ => return Err("Expected a map".to_owned()),
+        };
+
+        // Get the key, and verify that the value is the correct type. If so, push it.
+        if let Some(value) = map.get(key).and_then(|v| verifier(v)) {
+            out.push(value.clone());
+        }
+    }
+
+    Ok(out)
+}