bufio: add Writer.AvailableBuffer

This adds a new Writer.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 := bw.AvailableBuffer()
	b = appendValue(b, v)
	bw.Write(b)

It allows logic combining append-like APIs with bufio.Writer to avoid
needing to allocate and manage buffers themselves and allows the
append-like APIs to directly write into the buffer for a bufio.Writer.

Fixes #47527

Change-Id: I9cd169f3f8e8c7cd40818caf3daf1944c826fc66
Reviewed-on: https://go-review.googlesource.com/c/go/+/345569


Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>