Markup.ml 0.7.4

• Decrease parsing time of HTML parser by 20-95%, depending on properties of the
  input (#15, #16, af90cdf, e1d78e7). The XML parser seems to have benefitted as
  a side effect, with parsing times decreased by about 18%.
• Fix stack growth in Markup_lwt and deprecate Markup_lwt.ensure_tail_calls
  (9e0219d, 1e0a078).
• Prefix internal module names to avoid conflicts.
• Install .cmx files for internal modules, for Flambda switches.
• Add Markup_lwt.load.
• Move opam to the root directory for easier pinning.