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).

net.go

@@ -1,4 +1,4 @@
-// The following is copied from Go 1.18 official implementation.
+// TINYGO: The following is copied and modified from Go 1.19.3 official implementation.
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -7,7 +7,6 @@
 package net
 
 import (
-	"io"
 	"time"
 )
 
@@ -81,10 +80,6 @@ type Conn interface {
 	SetWriteDeadline(t time.Time) error
 }
 
-type conn struct {
-	//
-}
-
 // A Listener is a generic network listener for stream-oriented protocols.
 //
 // Multiple goroutines may invoke methods on a Listener simultaneously.
@@ -193,87 +188,3 @@ func (e *AddrError) Error() string {
 	}
 	return s
 }
-
-func (e *AddrError) Timeout() bool   { return false }
-func (e *AddrError) Temporary() bool { return false }
-
-// ErrClosed is the error returned by an I/O call on a network
-// connection that has already been closed, or that is closed by
-// another goroutine before the I/O is completed. This may be wrapped
-// in another error, and should normally be tested using
-// errors.Is(err, net.ErrClosed).
-var ErrClosed = errClosed
-
-// buffersWriter is the interface implemented by Conns that support a
-// "writev"-like batch write optimization.
-// writeBuffers should fully consume and write all chunks from the
-// provided Buffers, else it should report a non-nil error.
-type buffersWriter interface {
-	writeBuffers(*Buffers) (int64, error)
-}
-
-// Buffers contains zero or more runs of bytes to write.
-//
-// On certain machines, for certain types of connections, this is
-// optimized into an OS-specific batch write operation (such as
-// "writev").
-type Buffers [][]byte
-
-var (
-	_ io.WriterTo = (*Buffers)(nil)
-	_ io.Reader   = (*Buffers)(nil)
-)
-
-// WriteTo writes contents of the buffers to w.
-//
-// WriteTo implements io.WriterTo for Buffers.
-//
-// WriteTo modifies the slice v as well as v[i] for 0 <= i < len(v),
-// but does not modify v[i][j] for any i, j.
-func (v *Buffers) WriteTo(w io.Writer) (n int64, err error) {
-	if wv, ok := w.(buffersWriter); ok {
-		return wv.writeBuffers(v)
-	}
-	for _, b := range *v {
-		nb, err := w.Write(b)
-		n += int64(nb)
-		if err != nil {
-			v.consume(n)
-			return n, err
-		}
-	}
-	v.consume(n)
-	return n, nil
-}
-
-// Read from the buffers.
-//
-// Read implements io.Reader for Buffers.
-//
-// Read modifies the slice v as well as v[i] for 0 <= i < len(v),
-// but does not modify v[i][j] for any i, j.
-func (v *Buffers) Read(p []byte) (n int, err error) {
-	for len(p) > 0 && len(*v) > 0 {
-		n0 := copy(p, (*v)[0])
-		v.consume(int64(n0))
-		p = p[n0:]
-		n += n0
-	}
-	if len(*v) == 0 {
-		err = io.EOF
-	}
-	return
-}
-
-func (v *Buffers) consume(n int64) {
-	for len(*v) > 0 {
-		ln0 := int64(len((*v)[0]))
-		if ln0 > n {
-			(*v)[0] = (*v)[0][n:]
-			return
-		}
-		n -= ln0
-		(*v)[0] = nil
-		*v = (*v)[1:]
-	}
-}