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/lifecycle/Response.ts

@@ -2,6 +2,7 @@ import {Request} from './Request'
 import {ErrorWithContext, HTTPStatus, BehaviorSubject} from '../../util'
 import {ServerResponse} from 'http'
 import {HTTPCookieJar} from '../kernel/HTTPCookieJar'
+import {Readable} from 'stream'
 
 /**
  * Error thrown when the server tries to re-send headers after they have been sent once.
@@ -47,7 +48,7 @@ export class Response {
     private isBlockingWriteback = false
 
     /** The body contents that should be written to the response. */
-    public body = ''
+    public body: string | Buffer | Uint8Array | Readable = ''
 
     /**
      * Behavior subject fired right before the response content is written.
@@ -192,18 +193,29 @@ export class Response {
      * Write the headers and specified data to the client.
      * @param data
      */
-    public async write(data: unknown): Promise<void> {
+    public async write(data: string | Buffer | Uint8Array | Readable): Promise<void> {
         return new Promise<void>((res, rej) => {
             if ( !this.sentHeaders ) {
                 this.sendHeaders()
             }
-            this.serverResponse.write(data, error => {
-                if ( error ) {
-                    rej(error)
-                } else {
-                    res()
-                }
-            })
+
+            if ( data instanceof Readable ) {
+                data.pipe(this.serverResponse)
+                    .on('finish', () => {
+                        res()
+                    })
+                    .on('error', error => {
+                        rej(error)
+                    })
+            } else {
+                this.serverResponse.write(data, error => {
+                    if ( error ) {
+                        rej(error)
+                    } else {
+                        res()
+                    }
+                })
+            }
         })
     }
 
@@ -212,9 +224,14 @@ export class Response {
      */
     public async send(): Promise<void> {
         await this.sending$.next(this)
-        this.setHeader('Content-Length', String(this.body?.length ?? 0))
+
+        if ( !(this.body instanceof Readable) ) {
+            this.setHeader('Content-Length', String(this.body?.length ?? 0))
+        }
+
         await this.write(this.body ?? '')
         this.end()
+
         await this.sent$.next(this)
     }