A couple perf improvements

[MihaZupan]

Improves parsing perf a bit (around 2-4 %).

Logically separated in commits:

• Removing branches from various character checks (removing redundant conditions in calling code, updating helpers)
• A bit faster AutoLink and FencedCodeBlock matching
• Removed per-parsing allocation of delegate captures in AutoIdentifiers
• Using frozen collections in a few minor places

[xoofx]

Thanks for the perf improvements!