+NOTE:+ *THIS VERSION IS STILL EXPERIMENTAL*.   The API is VERY likely to
be changed before the final version.

Originally written by Rui Ueyama.  The original (pure Scheme) version
is kept as `peg-orig.scm` for a reference; it is cleaner than the current
optimized version so it'd be easier to understand.

`benchmark.scm` and `prof.scm` are for tuning.  They use a sample data file
`data/13tokyo.csv`, which is a public domain data provided by Japanese
Postal Service.
