Switch to builtin unsafe slice creation.

[happygiraffe]

In Go 1.17 and Go 1.20, new functions were added to create slices which significantly simplify the "reinterpret []byte" functions.

This should be a pure-refactoring. All tests pass.

This was surfaced while looking at an msan issue inside Google (b/461253460).

Benchmarks

I ran go test -bench='.*serial.*' -run=^$ before and after this change.
There appears to be no change in performance.

% benchstat /tmp/before /tmp/after
goos: darwin
goarch: arm64
pkg: github.com/RoaringBitmap/roaring/v2
cpu: Apple M1 Pro
                                            │ /tmp/before  │              /tmp/after              │
                                            │    sec/op    │    sec/op     vs base                │
UnserializeReadFrom/ReadFrom-650-10           323.3n ± ∞ ¹   322.3n ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500-10          1.201µ ± ∞ ¹   1.188µ ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-65000-10         1.182µ ± ∞ ¹   1.191µ ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-650000-10        1.577µ ± ∞ ¹   1.653µ ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500000-10       5.533µ ± ∞ ¹   5.562µ ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650-10       74.28n ± ∞ ¹   72.88n ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500-10      77.76n ± ∞ ¹   70.13n ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-65000-10     75.16n ± ∞ ¹   69.88n ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650000-10    335.5n ± ∞ ¹   310.8n ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500000-10   2.547µ ± ∞ ¹   2.409µ ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                       521.4n         507.0n        -2.77%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                                            │  /tmp/before  │              /tmp/after               │
                                            │     B/op      │     B/op       vs base                │
UnserializeReadFrom/ReadFrom-650-10           1.473Ki ± ∞ ¹   1.473Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500-10          8.109Ki ± ∞ ¹   8.109Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-65000-10         8.109Ki ± ∞ ¹   8.109Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-650000-10        8.613Ki ± ∞ ¹   8.613Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500000-10       13.68Ki ± ∞ ¹   13.68Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650-10         44.00 ± ∞ ¹     44.00 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500-10        52.00 ± ∞ ¹     52.00 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-65000-10       52.00 ± ∞ ¹     52.00 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650000-10      448.0 ± ∞ ¹     448.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500000-10   4.415Ki ± ∞ ¹   4.415Ki ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                       1.093Ki         1.093Ki        +0.00%
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

                                            │ /tmp/before │             /tmp/after              │
                                            │  allocs/op  │  allocs/op   vs base                │
UnserializeReadFrom/ReadFrom-650-10           8.000 ± ∞ ¹   8.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500-10          8.000 ± ∞ ¹   8.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-65000-10         8.000 ± ∞ ¹   8.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-650000-10        27.00 ± ∞ ¹   27.00 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeReadFrom/ReadFrom-6500000-10       207.0 ± ∞ ¹   207.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650-10       4.000 ± ∞ ¹   4.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500-10      4.000 ± ∞ ¹   4.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-65000-10     4.000 ± ∞ ¹   4.000 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-650000-10    13.00 ± ∞ ¹   13.00 ± ∞ ¹       ~ (p=1.000 n=1) ²
UnserializeFromBuffer/FromBuffer-6500000-10   103.0 ± ∞ ¹   103.0 ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                       13.77         13.77        +0.00%
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal

[lemire]

Looks like a no brainer. I'll hunt for someone to review this because I am not familiar with these new functions. (I trust you, but I don't trust myself.)

[clausecker]

Looks good to me.

[lemire]

Merging based on @clausecker's review.

[lemire]

Will be part of the next release.