mmap: implement io.WriterTo and io.ReadSeeker

[costela]

This extends the ReaderAt type to also implement io.WriterTo and
io.ReadSeeker, the later being required for the former.

Fixes golang/go#56422

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[costela]

Preliminary results from the included benchmark, which represents simple usage via io.Copy:

goos: linux
goarch: amd64
pkg: golang.org/x/exp/mmap
cpu: 12th Gen Intel(R) Core(TM) i7-1250U
            │  old.bench   │              new.bench              │
            │    sec/op    │   sec/op     vs base                │
MmapCopy-12   138.80µ ± 5%   65.80µ ± 2%  -52.60% (p=0.000 n=10)

            │  old.bench   │               new.bench               │
            │     B/s      │     B/s       vs base                 │
MmapCopy-12   15.17Gi ± 4%   32.00Gi ± 2%  +110.95% (p=0.000 n=10)

            │   old.bench   │             new.bench              │
            │     B/op      │    B/op     vs base                │
MmapCopy-12   32784.00 ± 0%   16.00 ± 0%  -99.95% (p=0.000 n=10)

            │ old.bench  │             new.bench              │
            │ allocs/op  │ allocs/op   vs base                │
MmapCopy-12   2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)

[gopherbot]

Message from Leo Antunes:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from qiulaidongfeng:

Patch Set 2: Run-TryBot+1

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[gopherbot]

Message from Gopher Robot:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 2: TryBot-Result+1

(1 comment)

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[gopherbot]

Message from qiulaidongfeng:

Patch Set 2: -Run-TryBot Code-Review+1

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[gopherbot]

This PR (HEAD: a3eda1e) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/exp/+/555796.

Important tips:

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