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/internal/ascii/print.go

@@ -0,0 +1,63 @@
+// TINYGO: The following is copied from Go 1.19.3 official implementation.
+
+// Copyright 2021 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 ascii
+
+import (
+	"strings"
+	"unicode"
+)
+
+// EqualFold is strings.EqualFold, ASCII only. It reports whether s and t
+// are equal, ASCII-case-insensitively.
+func EqualFold(s, t string) bool {
+	if len(s) != len(t) {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if lower(s[i]) != lower(t[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// lower returns the ASCII lowercase version of b.
+func lower(b byte) byte {
+	if 'A' <= b && b <= 'Z' {
+		return b + ('a' - 'A')
+	}
+	return b
+}
+
+// IsPrint returns whether s is ASCII and printable according to
+// https://tools.ietf.org/html/rfc20#section-4.2.
+func IsPrint(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] < ' ' || s[i] > '~' {
+			return false
+		}
+	}
+	return true
+}
+
+// Is returns whether s is ASCII.
+func Is(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] > unicode.MaxASCII {
+			return false
+		}
+	}
+	return true
+}
+
+// ToLower returns the lowercase version of s if s is ASCII and printable.
+func ToLower(s string) (lower string, ok bool) {
+	if !IsPrint(s) {
+		return "", false
+	}
+	return strings.ToLower(s), true
+}

http/internal/ascii/print_test.go

@@ -0,0 +1,95 @@
+// Copyright 2021 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 ascii
+
+import "testing"
+
+func TestEqualFold(t *testing.T) {
+	var tests = []struct {
+		name string
+		a, b string
+		want bool
+	}{
+		{
+			name: "empty",
+			want: true,
+		},
+		{
+			name: "simple match",
+			a:    "CHUNKED",
+			b:    "chunked",
+			want: true,
+		},
+		{
+			name: "same string",
+			a:    "chunked",
+			b:    "chunked",
+			want: true,
+		},
+		{
+			name: "Unicode Kelvin symbol",
+			a:    "chunKed", // This "K" is 'KELVIN SIGN' (\u212A)
+			b:    "chunked",
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := EqualFold(tt.a, tt.b); got != tt.want {
+				t.Errorf("AsciiEqualFold(%q,%q): got %v want %v", tt.a, tt.b, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestIsPrint(t *testing.T) {
+	var tests = []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{
+			name: "empty",
+			want: true,
+		},
+		{
+			name: "ASCII low",
+			in:   "This is a space: ' '",
+			want: true,
+		},
+		{
+			name: "ASCII high",
+			in:   "This is a tilde: '~'",
+			want: true,
+		},
+		{
+			name: "ASCII low non-print",
+			in:   "This is a unit separator: \x1F",
+			want: false,
+		},
+		{
+			name: "Ascii high non-print",
+			in:   "This is a Delete: \x7F",
+			want: false,
+		},
+		{
+			name: "Unicode letter",
+			in:   "Today it's 280K outside: it's freezing!", // This "K" is 'KELVIN SIGN' (\u212A)
+			want: false,
+		},
+		{
+			name: "Unicode emoji",
+			in:   "Gophers like 🧀",
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := IsPrint(tt.in); got != tt.want {
+				t.Errorf("IsASCIIPrint(%q): got %v want %v", tt.in, got, tt.want)
+			}
+		})
+	}
+}