Fix a few bugs, some message improvements

Bugs fixed:
- Had > instead of < when trying to check (< 1 length)
- Check that the list of benchmarks for each alternative is non-empty,
  as plot can't infer bounds for #f. An empty list of
  benchmarks for an alternative is usually due to incorrectly specifying
  alternatives or the standard benchmark (to compare against)

Enhancements:
- Improve error/info about whether the standard benchmark passed to
  render-benchmark-alts is being treated as a group name or benchmark
  name
- Add time parser for times of the form 'real ([0-9]+).([0-9])+'
  + benchmark/examples/linux-time.rkt uses this to with
      /usr/bin/time to benchmark the linux sleep command
- run-benchmarks now accepts lists of benchmark-one? and
  benchmark-group? in addition to benchmark-one? and benchmark-group?
Port example from users mailing list

about vector-like data structures, inlining, etc.
Small Makefile changes