#137: print usage when --help is requested

[RAprogramm]

Closes #137

The first command the README suggests to a newcomer, target/release/phie --help, exited with code 1 and printed File '--help' does not exist, because
parse_args took args[1] as a file name whatever it was.

Two tests were written first and both failed on master:
prints_usage_when_help_is_requested and its -h twin in tests/cli_test.rs.
Then run learned to recognise --help and -h and to answer with the usage
line, and the usage line itself moved into a small usage function so that the
no-arguments error and the help output cannot drift apart.

Behaviour now:

Command	Output	Exit code
phie --help	Usage: phie <file.phie>	0
phie -h	Usage: phie <file.phie>	0
phie	Usage: phie <file.phie> on stderr	1, as before
phie program.phie	the dataized result	0, as before

Verified locally: 243 tests pass including the two new ones, cargo fmt --check
is clean, cargo clippy --all-targets is clean, target/debug/fibonacci 7 10
still prints 21 and 210, reuse lint and typos are clean, and the built binary
was run by hand for all four cases in the table above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (6025413) to head (fbeaa3f).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   94.09%   94.15%   +0.05%     
==========================================
  Files          14       14              
  Lines         711      718       +7     
==========================================
+ Hits          669      676       +7     
  Misses         42       42              

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