feat(env): add lexical environment with frames

Sources/KiteCore/Environment.swift

@@ -0,0 +1,39 @@
+public final class Environment {
+    var vars: [String: Value]
+    let parent: Environment?
+
+    public init(parent: Environment? = nil) {
+        self.vars = [:]
+        self.parent = parent
+    }
+
+    func define(_ name: String, _ value: Value) {
+        vars[name] = value
+    }
+
+    func get(_ name: String) -> Value? {
+        var env: Environment? = self
+        while let e = env {
+            if let v = e.vars[name] { return v }
+            env = e.parent
+        }
+        let base = baseName(name)
+        if base != name { return get(base) }
+        return nil
+    }
+
+    @discardableResult
+    func set(_ name: String, _ value: Value) -> Bool {
+        var env: Environment? = self
+        while let e = env {
+            if e.vars[name] != nil {
+                e.vars[name] = value
+                return true
+            }
+            env = e.parent
+        }
+        let base = baseName(name)
+        if base != name { return set(base, value) }
+        return false
+    }
+}