Use vendored unsafe class instead of emitting IL

[tonyredondo]

Summary of changes

This PR remove emitting IL and replace it with the use of the vendored unsafe class.

[datadog-ddstaging]

Datadog Report

Branch report: tony/use-vendored-unsafe
Commit report: b08d201
Test service: dd-trace-dotnet

✅ 0 Failed, 306733 Passed, 1571 Skipped, 41m 6.7s Wall Time

[andrewlock]

Execution-Time Benchmarks Report ⏱️

Execution-time results for samples comparing the following branches/commits:

• This PR (5215)
• master

Execution-time benchmarks measure the whole time it takes to execute a program. And are intended to measure the one-off costs. Cases where the execution time results for the PR are worse than latest master results are shown in red. The following thresholds were used for comparing the execution times:

• Welch test with statistical test for significance of 5%
• Only results indicating a difference greater than 5% and 5 ms are considered.

Note that these results are based on a single point-in-time result for each branch. For full results, see the dashboard.

Graphs show the p99 interval based on the mean and StdDev of the test run, as well as the mean value of the run (shown as a diamond below the graph).

gantt
    title Execution time (ms) FakeDbCommand (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (74ms)  : 64, 85
     .   : milestone, 74,
    master - mean (75ms)  : 65, 86
     .   : milestone, 75,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (982ms)  : 962, 1001
     .   : milestone, 982,
    master - mean (989ms)  : 969, 1009
     .   : milestone, 989,

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gantt
    title Execution time (ms) FakeDbCommand (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (111ms)  : 107, 114
     .   : milestone, 111,
    master - mean (111ms)  : 107, 115
     .   : milestone, 111,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (718ms)  : 696, 741
     .   : milestone, 718,
    master - mean (710ms)  : 691, 728
     .   : milestone, 710,

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gantt
    title Execution time (ms) FakeDbCommand (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (95ms)  : 91, 99
     .   : milestone, 95,
    master - mean (93ms)  : 90, 96
     .   : milestone, 93,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (668ms)  : 642, 695
     .   : milestone, 668,
    master - mean (671ms)  : 646, 696
     .   : milestone, 671,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (188ms)  : 184, 191
     .   : milestone, 188,
    master - mean (188ms)  : 184, 191
     .   : milestone, 188,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (1,062ms)  : 1038, 1086
     .   : milestone, 1062,
    master - mean (1,057ms)  : 1033, 1082
     .   : milestone, 1057,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (270ms)  : 265, 275
     .   : milestone, 270,
    master - mean (271ms)  : 266, 275
     .   : milestone, 271,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (1,057ms)  : 1033, 1081
     .   : milestone, 1057,
    master - mean (1,055ms)  : 1024, 1085
     .   : milestone, 1055,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (5215) - mean (259ms)  : 255, 263
     .   : milestone, 259,
    master - mean (259ms)  : 253, 264
     .   : milestone, 259,

    section CallTarget+Inlining+NGEN
    This PR (5215) - mean (999ms)  : 976, 1022
     .   : milestone, 999,
    master - mean (994ms)  : 967, 1022
     .   : milestone, 994,

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