perf: Optimize read MIME operation

[filzrev]

What included in this PR

• Set FileStream's BufferSize to 0 (Default:4096)
• Set StreamReader's BufferSize to 128 (Default:1024, Min:128)
• Set FileOptions.SequentialScan hint for Windows (It seems don't affects actual performance thought)

Background
YamlMime.ReadMime API read file's first line only.
So it can reduce internal buffer allocations. and reduce Gen0 GC allocations.

Benchmark results

Method	Mean	Error	StdDev	Ratio	Gen0	Gen1	Allocated	Alloc Ratio
ReadMime	19.23 μs	0.192 μs	0.180 μs	1.00	1.8616	0.0305	7.73 KB	1.00
ReadMime_Optimized	18.16 μs	0.083 μs	0.065 μs	0.94	0.2441	0.0305	1.02 KB	0.13

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (277aa86) 77.58% compared to head (06e6704) 77.59%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9132      +/-   ##
==========================================
+ Coverage   77.58%   77.59%   +0.01%     
==========================================
  Files         591      591              
  Lines       24567    24578      +11     
==========================================
+ Hits        19060    19071      +11     
  Misses       5507     5507              

Files Changed	Coverage Δ
src/Docfx.Common/YamlMime.cs	100.00% <100.00%> (ø)

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[KalleOlaviNiemitalo]

IIUC, [NotNull] on the parameter means that, if reader were null, then ReadMime would throw rather than return. So that if another method calls ReadMime and passes a possibly-null value, then the compiler can assume the value is not null after ReadMime returns.

Is that the purpose for which you use [NotNull] here?

[filzrev]

As you pointed out. [NotNull] is not works as expected.
In Anyway it's public API. parameter's null checks are required.
So I've remove [NotNull] attribute and revert original null check codes.

[yufeih]

Thank you @filzrev !