A handful of codegen improvements

[MihaZupan]

• Added a PeekChar() overload of PeekChar(int) to ICharIterator. This method was already present on StringSlice, but since it was missing on the interface, callers were often binding to the slower overload
• Added SkipChar to ICharIterator
• Replaced CurrentCharacter == '\0' and Length == 0 with faster IsEmpty checks
• Avoided a bunch of struct copies in StringLineGroup
• Changed the NewLine.Length() to a bit operation
• Simplified/optimized logic in HtmlHelper
• Avoided the ToUtf32 iterator allocations in Globalization extension

[iamcarbon]

Consider renaming SkipChar to Advance() and adding an Advance(count) overload to match SequenceReader naming.