Remove roSingleSegment arena

[lthibault]

Stacks onto #468. Merge that first.

This is a precursor to pooling segmets of MultiSegmentArena, as per discussion on Matrix.

Pooled messages obviate the need for message reuse in the Decoder, greatly simplifying its implementation. Care has been taken to avoid an extra copy where demuxArena was formerly called.

Benchmarks:

The performance degradation is due to MultiSegment arenas not drawn from the pool. This will be fixed in a subsequent PR.

Tested on Go version 1.20

Old

goos: darwin
goarch: amd64
pkg: capnproto.org/go/capnp/v3
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkDecode-8   	     180	   5625283 ns/op	 170.66 MB/s	11193828 B/op	   40022 allocs/op
PASS
ok  	capnproto.org/go/capnp/v3	1.948s

New

goos: darwin
goarch: amd64
pkg: capnproto.org/go/capnp/v3
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkDecode-8	171	13576764 ns/op	70.71 MB/s	12409647 B/op	50021 allocs/op
PASS
ok  	capnproto.org/go/capnp/v3	3.257s

[lthibault]

I'm going to re-open this as another PR.