Adapt to latest xhtml version, various optimizations

• xhtml now uses Builder and Text for an internal representation. This improves the memory performance of haddock. This MR adapts to the latest xhtml interface and updates the version bounds for xhtml.
• Reset default ghc verbosity to 0, and suggest --optghc=v1 to observe load' progress.
• Better verbosity flag handling and documentation
• More debug output for -v3
• Parallelize html generation
• Parallelize and fix a serious performance regression in JSON index generation (see here)