Add very rudimentary Lexer: it matches parens!

Sibil/Lexer.swift

@@ -0,0 +1,52 @@
+//
+//  Lexer.swift
+//  Sibil
+//
+//  Created by Eryn Wells on 12/16/16.
+//  Copyright © 2016 Eryn Wells. All rights reserved.
+//
+
+import Foundation
+
+
+struct Token {
+    enum Kind {
+        case LeftParen
+        case RightParen
+    }
+    
+    let kind: Kind
+    let value: String
+}
+
+
+class Lexer: IteratorProtocol {
+    typealias Element = Token
+
+    let input: String
+
+    private var index: String.Index
+
+    init(input: String) {
+        self.input = input
+        self.index = input.startIndex
+    }
+
+    // MARK: IteratorProtocol
+
+    func next() -> Token? {
+        var token: Token?
+        while token == nil {
+            let c = input[index]
+            switch c {
+            case "(":
+                token = Token(kind: .LeftParen, value: String(c))
+            case ")":
+                token = Token(kind: .RightParen, value: String(c))
+            default:
+                break
+            }
+        }
+        return token
+    }
+}