Fix minor typos.
Make internal index an ArrayRef instead of HashRef to reduce memory usage

This reduces a lot the memory used by the internal index.
Add xt/memory_usage.pl to benchmark differences with Slurp
reflect Changes
Add a BENCHMARK section in POD to explain about the memory consumption
reflect Changes
Remove HashRef mention in doc
typo in documentation
update README.pod
v0.003

    * Use an array instead of a hash for the internal index to reduce memory
      usage (up to 20x less memory consuming than File::Slurp with big files,
      which is 5 times better than previous versions, see README.pod).