Check that benchmark results comparable

- Prior to normalization, check that benchmark results are comparable
Test to compare racket's 'time' and /usr/bin/time
Reuse maybe-execute-cmd for running external bench
Add set vs list example
Documentation CP
Documentation CP
Update examples to not use `bench-group "" ...`

- run-benchmarks now supports (listof (or/c benchmark-one? benchmark-group?)
Documentation CP
Add variable to find all source files

- This is to be used for updating all of the files when an identifier
  changes
Rename a couple of thing

- time-external -> manual-report-time
- default-extract-time -> racket-extract-time
Make examples/set.rkt pass (renamed identifiers)
Fix ignored directories in Makefile
Rename racket-extract-time

- Now racket-time-extract-result (to be consistent with
  linux-time-extract-result)
- Add some more examples to documentation
Missed external.rkt when renaming time-external
Fix typo
Algorithms with different asymptotic complexities

- O(n) fib compared with O(2^n) fib
Export racket-time-extract-result for docs
Documentation CP
Factor main.rkt into exports and other functions

- run.rkt now has append-opts and run-benchmarks
- main.rkt now only has exports for the library
Documentation CP