Use xhp for html rendering

[lexidor]

fixes #1 fixes #14

Fixes:

• Remove dead fixmes.
• Lint with hhast lint.
• Introduces IRenderer<+T> for covariant handling of Renderer<T>.
• Introduces HTMLXHPRenderer to render straight to XHP.
• Introduces HTMLWithXHPInternallyRenderer to grant XHP benefits to users unable to use the xhp variant.