Replace font-lock with low-level implementation

[taku0]

The fontification logic is changed from a syntax table based to a text based implementation.

Benchmarks:

bench-jsonian on the main branch: 8.572s
bench-jsonian on the new branch: 4.204s
bench-prog: 5.794s
bench-fundamental: 5.862s
bench-fundamental-keyword-only: 0.105s

When font-lock-keywords-only is nil, font-lock-fontify-region took more than 5 seconds even in the fundamental mode. If it is non-nil, font-lock-fontify-region is instantaneous.

Drawbacks:

• Since fontification doesn't trigger syntax-ppss, some functions like forward-sexp or end-of-line took a fraction of a second (e.g. 0.37s) for the first time.
• It adds 157 lines of code and complicates jsonian.
• It may contain bugs (I hope not).

Bonus:

• Highlighting of escape sequences. "This\nmakes\nstrings\nlike\nthis\neasier\nto\nread."

Discussion:

Does this offer real benefit? Emacs' font locking is smart enough to fontify only a fraction of the buffer, so opening a large file or moving to the end of the buffer is instantaneous (unless the contents of the file is a single long line). Can we justify a long and complex implementation?