Add network device driver model, netdev

This PR adds a network device driver model called netdev. There will be a companion PR for TinyGo drivers to update the netdev drivers and network examples. This PR covers the core "net" package.

An RFC for the work is here: #tinygo-org/drivers#487. Some things have changed from the RFC, but nothing major.

The "net" package is a partial port of Go's "net" package, version 1.19.3. The src/net/README file has details on what is modified from Go's "net" package.

Most "net" features are working as they would in normal Go. TCP/UDP/TLS protocol support is there. As well as HTTP client and server support. Standard Go network packages such as golang.org/x/net/websockets and Paho MQTT client work as-is. Other packages are likely to work as-is.

Testing results are here (https://docs.google.com/spreadsheets/d/e/2PACX-1vT0cCjBvwXf9HJf6aJV2Sw198F2ief02gmbMV0sQocKT4y4RpfKv3dh6Jyew8lQW64FouZ8GwA2yjxI/pubhtml?gid=1013173032&single=true).

http/clone.go

@@ -0,0 +1,76 @@
+// TINYGO: The following is copied from Go 1.19.3 official implementation.
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package http
+
+import (
+	"mime/multipart"
+	"net/textproto"
+	"net/url"
+)
+
+func cloneURLValues(v url.Values) url.Values {
+	if v == nil {
+		return nil
+	}
+	// http.Header and url.Values have the same representation, so temporarily
+	// treat it like http.Header, which does have a clone:
+	return url.Values(Header(v).Clone())
+}
+
+func cloneURL(u *url.URL) *url.URL {
+	if u == nil {
+		return nil
+	}
+	u2 := new(url.URL)
+	*u2 = *u
+	if u.User != nil {
+		u2.User = new(url.Userinfo)
+		*u2.User = *u.User
+	}
+	return u2
+}
+
+func cloneMultipartForm(f *multipart.Form) *multipart.Form {
+	if f == nil {
+		return nil
+	}
+	f2 := &multipart.Form{
+		Value: (map[string][]string)(Header(f.Value).Clone()),
+	}
+	if f.File != nil {
+		m := make(map[string][]*multipart.FileHeader)
+		for k, vv := range f.File {
+			vv2 := make([]*multipart.FileHeader, len(vv))
+			for i, v := range vv {
+				vv2[i] = cloneMultipartFileHeader(v)
+			}
+			m[k] = vv2
+		}
+		f2.File = m
+	}
+	return f2
+}
+
+func cloneMultipartFileHeader(fh *multipart.FileHeader) *multipart.FileHeader {
+	if fh == nil {
+		return nil
+	}
+	fh2 := new(multipart.FileHeader)
+	*fh2 = *fh
+	fh2.Header = textproto.MIMEHeader(Header(fh.Header).Clone())
+	return fh2
+}
+
+// cloneOrMakeHeader invokes Header.Clone but if the
+// result is nil, it'll instead make and return a non-nil Header.
+func cloneOrMakeHeader(hdr Header) Header {
+	clone := hdr.Clone()
+	if clone == nil {
+		clone = make(Header)
+	}
+	return clone
+}