File-based response support & static server

- Clean up UniversalPath implementation
    - Use Readable/Writable types correctly for stream methods
    - Add .list() methods for getting child files

- Make Response body specify explicit types and support
  writing Readable streams to the body

- Create a static file server that supports directory listing

src/http/response/FileResponseFactory.ts

@@ -0,0 +1,36 @@
+import {ResponseFactory} from './ResponseFactory'
+import {Request} from '../lifecycle/Request'
+import {ErrorWithContext, UniversalPath} from '../../util'
+
+/**
+ * Helper function that creates a FileResponseFactory for the given path.
+ * @param path
+ */
+export function file(path: UniversalPath): FileResponseFactory {
+    return new FileResponseFactory(path)
+}
+
+/**
+ * HTTP response factory that sends a file referenced by a given UniversalPath.
+ */
+export class FileResponseFactory extends ResponseFactory {
+    constructor(
+        /** The file to be sent. */
+        public readonly path: UniversalPath,
+    ) {
+        super()
+    }
+
+    public async write(request: Request): Promise<Request> {
+        if ( !(await this.path.isFile()) ) {
+            throw new ErrorWithContext(`Cannot write non-file resource as response: ${this.path}`, {
+                path: this.path,
+            })
+        }
+
+        request.response.setHeader('Content-Type', this.path.contentType || 'application/octet-stream')
+        request.response.setHeader('Content-Length', String(await this.path.sizeInBytes()))
+        request.response.body = await this.path.readStream()
+        return request
+    }
+}