April perf improvements

[MihaZupan]

Interesting changes:

• Reducing the size of AST objects (mainly Inlines)
• Reducing the cost of type checks by using spare bits on MarkdownObject to store the type and then doing Unsafe casts
• Avoiding substring allocations to store CodeInline.Content
• Adding various fast-paths to parsing for the advanced pipeline
• Faster rendering loop by replacing Type.GetTypeHandle with the method table pointer

Perf:

Default pipeline

Parsing (~5% speedup, ~8% fewer allocated bytes)

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
Parse	MarkdigReadme	279.842 us	0.8528 us	1.2231 us	280.632 us	139,241 B
Parse	TracingArticle	37.482 us	0.0957 us	0.1373 us	37.439 us	18,592 B
Parse	YarpDocs	2,727.076 us	13.5193 us	19.3889 us	2,740.968 us	1,320,172 B

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
Parse	MarkdigReadme	264.739 us	1.1761 us	1.7240 us	264.500 us	127,737 B
Parse	TracingArticle	36.518 us	0.1381 us	0.2024 us	36.614 us	17,088 B
Parse	YarpDocs	2,593.829 us	13.2596 us	19.8464 us	2,594.943 us	1,218,524 B

Rendering (2-10% speedup)

Method	SourceTexts	Mean	Error	StdDev	Median
RenderWithoutToString	MarkdigReadme	62.725 us	0.6066 us	0.8891 us	63.415 us
RenderWithoutToString	TracingArticle	8.028 us	0.0060 us	0.0088 us	8.028 us
RenderWithoutToString	YarpDocs	825.007 us	6.1494 us	9.0137 us	817.198 us

Method	SourceTexts	Mean	Error	StdDev	Median
RenderWithoutToString	MarkdigReadme	61.584 us	1.1683 us	1.5992 us	62.792 us
RenderWithoutToString	TracingArticle	7.328 us	0.0051 us	0.0077 us	7.329 us
RenderWithoutToString	YarpDocs	786.404 us	4.9851 us	7.3071 us	791.788 us

Advanced pipeline

Parsing (7-16% speedup, ~7% fewer allocated bytes)

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
Parse	MarkdigReadme	414.583 us	2.2855 us	3.4208 us	413.392 us	153,857 B
Parse	TracingArticle	69.383 us	0.5513 us	0.7907 us	69.452 us	23,288 B
Parse	YarpDocs	6,137.069 us	38.8496 us	51.8631 us	6,175.543 us	2,199,560 B

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
Parse	MarkdigReadme	387.088 us	11.0335 us	16.5144 us	380.891 us	142,273 B
Parse	TracingArticle	61.665 us	0.0889 us	0.1217 us	61.675 us	21,784 B
Parse	YarpDocs	5,141.475 us	13.8774 us	20.3413 us	5,148.576 us	2,062,000 B

Rendering

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
RenderWithoutToString	MarkdigReadme	72.981 us	0.2856 us	0.3909 us	73.300 us	1,944 B
RenderWithoutToString	TracingArticle	8.533 us	0.0166 us	0.0232 us	8.529 us	-
RenderWithoutToString	YarpDocs	975.213 us	2.5721 us	3.7701 us	977.482 us	2,746 B

Method	SourceTexts	Mean	Error	StdDev	Median	Allocated
RenderWithoutToString	MarkdigReadme	69.082 us	0.0590 us	0.0864 us	69.055 us	1,944 B
RenderWithoutToString	TracingArticle	7.807 us	0.0147 us	0.0215 us	7.809 us	-
RenderWithoutToString	YarpDocs	917.742 us	2.4117 us	3.5351 us	918.906 us	2,745 B

[xoofx]

Great job, thanks!