feat(cli): add repl and file runner entry point

Sources/kite/main.swift

@@ -0,0 +1,67 @@
+import Foundation
+import KiteCore
+
+func runFile(_ path: String) -> Int32 {
+    guard let source = try? String(contentsOfFile: path, encoding: .utf8) else {
+        FileHandle.standardError.write(Data("kite: cannot read file: \(path)\n".utf8))
+        return 1
+    }
+    let interp = Interpreter()
+    do {
+        try interp.evalString(source)
+        return 0
+    } catch let error as SchemeError {
+        FileHandle.standardError.write(Data("error: \(error.description)\n".utf8))
+        return 1
+    } catch {
+        FileHandle.standardError.write(Data("error: \(error)\n".utf8))
+        return 1
+    }
+}
+
+func runRepl() {
+    let interp = Interpreter()
+    print("kite - a Scheme interpreter. Ctrl-D to exit.")
+    var buffer = ""
+    while true {
+        if buffer.isEmpty {
+            print("kite> ", terminator: "")
+        } else {
+            print("...   ", terminator: "")
+        }
+        guard let line = readLine(strippingNewline: false) else {
+            print("")
+            break
+        }
+        buffer += line
+        let forms: [Value]
+        do {
+            forms = try Reader(buffer).readProgram()
+        } catch {
+            continue
+        }
+        buffer = ""
+        for form in forms {
+            do {
+                let result = try interp.evalTop(form)
+                switch result {
+                case .unspecified:
+                    break
+                default:
+                    print(writeString(result))
+                }
+            } catch let error as SchemeError {
+                print("error: \(error.description)")
+            } catch {
+                print("error: \(error)")
+            }
+        }
+    }
+}
+
+let arguments = CommandLine.arguments
+if arguments.count > 1 {
+    exit(runFile(arguments[1]))
+} else {
+    runRepl()
+}