A few more perf improvements

[MihaZupan]

• Improves the fenced code block parser a bit
• Avoids a dictionary lookup per object when rendering (using linear scans over small arrays instead)
• Misc

Method	Job	Mean	Error	Ratio
Parse	main	2,512.1 us	13.63 us	1.00
Parse	pr	2,417.2 us	14.36 us	0.96
ParseAdvanced	main	7,872.1 us	22.51 us	1.00
ParseAdvanced	pr	7,906.6 us	44.54 us	1.00
Render	main	716.3 us	2.38 us	1.00
Render	pr	688.6 us	4.58 us	0.96
RenderAdvanced	main	1,073.6 us	4.24 us	1.00
RenderAdvanced	pr	984.2 us	6.25 us	0.92

[coveralls]

Pull Request Test Coverage Report for Build 6993805425

• 95 of 105 (90.48%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.006%) to 93.137%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig/Helpers/EntityHelper.cs	4	9	44.44%
src/Markdig/Helpers/UnicodeUtility.cs	4	9	44.44%

Files with Coverage Reduction	New Missed Lines	%
src/Markdig/Helpers/EntityHelper.cs	1	99.13%

Totals
Change from base Build 6988197475:	0.006%
Covered Lines:	25990
Relevant Lines:	27274

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💛 - Coveralls