switch to slog.Logger for logging, for easier reuse of packages by external software

we don't want external software to include internal details like mlog.
slog.Logger is/will be the standard.

we still have mlog for its helper functions, and its handler that logs in
concise logfmt used by mox.

packages that are not meant for reuse still pass around mlog.Log for
convenience.

we use golang.org/x/exp/slog because we also support the previous Go toolchain
version. with the next Go release, we'll switch to the builtin slog.

vendor/golang.org/x/exp/slog/internal/buffer/buffer.go

@@ -0,0 +1,84 @@
+// Copyright 2022 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 buffer provides a pool-allocated byte buffer.
+package buffer
+
+import (
+	"sync"
+)
+
+// Buffer adapted from go/src/fmt/print.go
+type Buffer []byte
+
+// Having an initial size gives a dramatic speedup.
+var bufPool = sync.Pool{
+	New: func() any {
+		b := make([]byte, 0, 1024)
+		return (*Buffer)(&b)
+	},
+}
+
+func New() *Buffer {
+	return bufPool.Get().(*Buffer)
+}
+
+func (b *Buffer) Free() {
+	// To reduce peak allocation, return only smaller buffers to the pool.
+	const maxBufferSize = 16 << 10
+	if cap(*b) <= maxBufferSize {
+		*b = (*b)[:0]
+		bufPool.Put(b)
+	}
+}
+
+func (b *Buffer) Reset() {
+	*b = (*b)[:0]
+}
+
+func (b *Buffer) Write(p []byte) (int, error) {
+	*b = append(*b, p...)
+	return len(p), nil
+}
+
+func (b *Buffer) WriteString(s string) {
+	*b = append(*b, s...)
+}
+
+func (b *Buffer) WriteByte(c byte) {
+	*b = append(*b, c)
+}
+
+func (b *Buffer) WritePosInt(i int) {
+	b.WritePosIntWidth(i, 0)
+}
+
+// WritePosIntWidth writes non-negative integer i to the buffer, padded on the left
+// by zeroes to the given width. Use a width of 0 to omit padding.
+func (b *Buffer) WritePosIntWidth(i, width int) {
+	// Cheap integer to fixed-width decimal ASCII.
+	// Copied from log/log.go.
+
+	if i < 0 {
+		panic("negative int")
+	}
+
+	// Assemble decimal in reverse order.
+	var bb [20]byte
+	bp := len(bb) - 1
+	for i >= 10 || width > 1 {
+		width--
+		q := i / 10
+		bb[bp] = byte('0' + i - q*10)
+		bp--
+		i = q
+	}
+	// i < 10
+	bb[bp] = byte('0' + i)
+	b.Write(bb[bp:])
+}
+
+func (b *Buffer) String() string {
+	return string(*b)
+}

vendor/golang.org/x/exp/slog/internal/ignorepc.go

@@ -0,0 +1,9 @@
+// Copyright 2023 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 internal
+
+// If IgnorePC is true, do not invoke runtime.Callers to get the pc.
+// This is solely for benchmarking the slowdown from runtime.Callers.
+var IgnorePC = false