Some CPU improvements

[MihaZupan]

• Caching the FencedCodeBlock.InfoString & language-prefixed strings
• Using ValueStringBuilder over StringBuilderCache.Local()
• Avoiding the cost of type checks & casting for ContainerBlock, LeafBlock, ParagraphBlock and ContainerInline by setting flags on the base type instead (this is a ~5% CPU win by removing S.R.CompilerServices.CastHelpers.IsInstanceOfClass).
• Improved StringLineGroup.NextChar - very hot method for LinkHelper

Parsing this document with PreciseSourceLocation:

Before

Method	SourceText	Mean	Error	StdDev	Gen 0	Allocated
Parse	TracingArticle	42.77 us	0.232 us	0.332 us	4.4556	18 KB

After

Method	SourceText	Mean	Error	StdDev	Gen 0	Allocated
Parse	TracingArticle	37.39 us	0.215 us	0.309 us	4.3945	18 KB

[coveralls]

Pull Request Test Coverage Report for Build 1980002829

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 523 of 546 (95.79%) changed or added relevant lines in 27 files are covered.
• 13 unchanged lines in 3 files lost coverage.
• Overall coverage remained the same at 93.053%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig/Helpers/StringLineGroup.cs	70	71	98.59%
src/Markdig/Helpers/LinkHelper.cs	57	59	96.61%
src/Markdig/Helpers/StringSlice.cs	7	9	77.78%
src/Markdig/Parsers/BlockProcessor.cs	33	35	94.29%
src/Markdig/Helpers/TransformedStringCache.cs	75	79	94.94%
src/Markdig/Helpers/ValueStringBuilder.cs	96	108	88.89%

Files with Coverage Reduction	New Missed Lines	%
src/Markdig/Helpers/LinkHelper.cs	2	91.55%
src/Markdig/Helpers/StringLineGroup.cs	3	94.37%
src/Markdig/Helpers/StringBuilderCache.cs	8	0.0%

Totals
Change from base Build 1979509409:	0.0%
Covered Lines:	25100
Relevant Lines:	26343

---

💛 - Coveralls

[xoofx]

Thanks a lot, amazing numbers!