bytes: add Buffer.Available and Buffer.AvailableBuffer

This adds a new Buffer.AvailableBuffer method that returns
an empty buffer with a possibly non-empty capacity for use
with append-like APIs.

The typical usage pattern is something like:

	b := bb.AvailableBuffer()
	b = appendValue(b, v)
	bb.Write(b)

It allows logic combining append-like APIs with Buffer
to avoid needing to allocate and manage buffers themselves and
allows the append-like APIs to directly write into the Buffer.

The Buffer.Write method uses the builtin copy function,
which avoids copying bytes if the source and destination are identical.
Thus, Buffer.Write is a constant-time call for this pattern.

Performance:

	BenchmarkBufferAppendNoCopy  2.909 ns/op  5766942167.24 MB/s

This benchmark should only be testing the cost of bookkeeping
and never the copying of the input slice.
Thus, the MB/s should be orders of magnitude faster than RAM.

Fixes #53685

Change-Id: I0b41e54361339df309db8d03527689b123f99085
Reviewed-on: https://go-review.googlesource.com/c/go/+/474635
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/bytes/buffer.go

@@ -53,6 +53,12 @@ const maxInt = int(^uint(0) >> 1)
 // so immediate changes to the slice will affect the result of future reads.
 func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
 
+// AvailableBuffer returns an empty buffer with b.Available() capacity.
+// This buffer is intended to be appended to and
+// passed to an immediately succeeding Write call.
+// The buffer is only valid until the next write operation on b.
+func (b *Buffer) AvailableBuffer() []byte { return b.buf[len(b.buf):] }
+
 // String returns the contents of the unread portion of the buffer
 // as a string. If the Buffer is a nil pointer, it returns "<nil>".
 //
@@ -76,6 +82,9 @@ func (b *Buffer) Len() int { return len(b.buf) - b.off }
 // total space allocated for the buffer's data.
 func (b *Buffer) Cap() int { return cap(b.buf) }
 
+// Available returns how many bytes are unused in the buffer.
+func (b *Buffer) Available() int { return cap(b.buf) - len(b.buf) }
+
 // Truncate discards all but the first n unread bytes from the buffer
 // but continues to use the same allocated storage.
 // It panics if n is negative or greater than the length of the buffer.