Update Cloudflare createHash polyfill to support md5 and hex encoding

Since the md5 method in cf/src/connection.js expects to be able to call
crypto.createHash('md5').update(x).digest('hex')

This was causing md5 password auth to hang when used from a Cloudflare
worker, but now I've confirmed md5 password auth works.

cf/polyfills.js

@@ -47,12 +47,25 @@ export const crypto = {
     ),
   createHash: type => ({
     update: x => ({
-      digest: () => {
-        if (type !== 'sha256')
-          throw Error('createHash only supports sha256 in this environment.')
-        if (!(x instanceof Uint8Array))
+      digest: encoding => {
+        if (!(x instanceof Uint8Array)) {
           x = textEncoder.encode(x)
-        return Crypto.subtle.digest('SHA-256', x)
+        }
+        let prom
+        if (type === 'sha256') {
+          prom = Crypto.subtle.digest('SHA-256', x)
+        } else if (type === 'md5') {
+          prom = Crypto.subtle.digest('md5', x)
+        } else {
+          throw Error('createHash only supports sha256 or md5 in this environment, not ${type}.')
+        }
+        if (encoding === 'hex') {
+          return prom.then((arrayBuf) => Buffer.from(arrayBuf).toString('hex'))
+        } else if (encoding) {
+          throw Error(`createHash only supports hex encoding or unencoded in this environment, not ${encoding}`)
+        } else {
+          return prom
+        }
       }
     })
   }),