Replace gperf-based entity lookup with binary tree lookup.

The primary advantage is a big reduction in the size of
the compiled library and executable (> 100K).

There should be no measurable performance difference in
normal documents.  I detected a slight performance
hit (around 5%) in a file containing 1,000,000 entities.

* Removed `src/html_unescape.gperf` and `src/html_unescape.h`.
* Added `src/entities.h` (generated by `tools/make_entities_h.py`).
* Added binary tree lookup functions to `houdini_html_u.c`, and
  use the data in `src/entities.h`.

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