[DO-NOT-MERGE] [buffer] Make unsafe a few hot callsites

[behdad]

For illustration / discussion only.

This uses unchecked_get[_mut] in a few hotspots of the buffer code. Most of them can easily be reasoned as always correct by checking the calling code for loop bounds.

I don't see as much speedup as I was hoping. Also, I have no idea why Gulzar slows down, but it seems to do consistently:

behdad:hb 0 (main)$ subprojects/benchmark-1.8.4/tools/compare.py benchmarks before after
Comparing before to after
Benchmark                                                                               Time             CPU      Time Old      Time New       CPU Old       CPU New
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
BM_Shape/Roboto-Regular.ttf/en-thelittleprince.txt/harfrust                          -0.0458         -0.0453             7             7             7             7
BM_Shape/Roboto-Regular.ttf/en-words.txt/harfrust                                    -0.0325         -0.0363            12            12            12            11
BM_Shape/SourceSerifVariable-Roman.ttf/react-dom.txt/harfrust                        -0.0266         -0.0269            77            75            76            74
BM_Shape/NotoSansDevanagari-Regular.ttf/hi-words.txt/harfrust                        -0.0295         -0.0301            27            26            27            26
BM_Shape/Amiri-Regular.ttf/fa-thelittleprince.txt/harfrust                           -0.0281         -0.0280            39            38            39            38
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-thelittleprince.txt/harfrust                -0.0140         -0.0140            87            86            87            86
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-words.txt/harfrust                          -0.0195         -0.0186           104           102           104           102
BM_Shape/Gulzar-Regular.ttf/fa-thelittleprince.txt/harfrust                          +0.0412         +0.0414          2215          2306          2204          2296
BM_Shape/Gulzar-Regular.ttf/fa-words.txt/harfrust                                    +0.0339         +0.0339          1429          1478          1422          1470
BM_Shape/NotoSansDuployan-Regular.otf/duployan.txt/harfrust                          -0.0422         -0.0424          2928          2804          2915          2792
OVERALL_GEOMEAN                                                                      -0.0167         -0.0170             0             0             0             0

[behdad]

Note that if I run this patch on top of the unchecked-read fontations changes, then I get higher boost indeed:

Comparing before to after
Benchmark                                                                               Time             CPU      Time Old      Time New       CPU Old       CPU New
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
BM_Shape/Roboto-Regular.ttf/en-thelittleprince.txt/harfrust                          -0.0777         -0.0769             7             6             7             6
BM_Shape/Roboto-Regular.ttf/en-words.txt/harfrust                                    -0.0413         -0.0412            12            11            12            11
BM_Shape/SourceSerifVariable-Roman.ttf/react-dom.txt/harfrust                        -0.0557         -0.0556            72            68            72            68
BM_Shape/NotoSansDevanagari-Regular.ttf/hi-words.txt/harfrust                        -0.0275         -0.0280            26            26            26            25
BM_Shape/Amiri-Regular.ttf/fa-thelittleprince.txt/harfrust                           -0.0117         -0.0141            32            32            32            32
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-thelittleprince.txt/harfrust                -0.0453         -0.0453            61            59            61            58
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-words.txt/harfrust                          -0.0267         -0.0270            74            72            74            72
BM_Shape/Gulzar-Regular.ttf/fa-thelittleprince.txt/harfrust                          +0.0110         +0.0109          1288          1303          1282          1296
BM_Shape/Gulzar-Regular.ttf/fa-words.txt/harfrust                                    +0.0056         +0.0058           830           834           825           830
BM_Shape/NotoSansDuployan-Regular.otf/duployan.txt/harfrust                          -0.0894         -0.0892          1879          1711          1871          1704
OVERALL_GEOMEAN                                                                      -0.0364         -0.0366             0             0             0             0

[behdad]

@dfrg @rsheeter These kinds of guaranteed correct unsafes, are these kosher?

[behdad]

Needs inspecting the compiler output to make sure it makes any difference first.