[clap-v3-utils] Add try_get_word_count

[samkim-crypto]

Problem

With clap-v3, the value_of, value_of_t, and other value_of_... functions have been deprecated. These functions were convenient in that they performed a type conversion on the parsed string argument. The function that replaces these functions get_one::<T>(...) can also perform type conversion by specifying a generic type T; unfortunately, the need to specify T as a concrete type makes usage less flexible.

For example, consider the word count argument.

pub fn word_count_arg<'a>() -> Arg<'a> {
    Arg::new(WORD_COUNT_ARG.name)
        .long(WORD_COUNT_ARG.long)
        .value_parser(PossibleValuesParser::new(["12", "15", "18", "21", "24"]))
        .default_value("12")
        .value_name("NUMBER")
        .takes_value(true)
        .help(WORD_COUNT_ARG.help)
}

With the possible values specified as strings, calling get_one::<usize>(WORD_COUNT_ARG.name) will produce a downcast error since we have not specified how to convert each string to usize.

The canonical way to take care of this in clap-v3 is to create an enum WordCount { ... }, implement Into<usize> and call let var: usize = get_one::<WordCount>(...).into().

Alternatively, we can let caller take care of the string conversion get_one::<String>(...).map(|word_count| word_count.parse::<usize>()).

Both methods seem to produce unnecessary and inelegant code.

Summary of Changes

Add a function try_get_word_count that can directly parse usize from arg matches.

Fixes #

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 81.9%. Comparing base (b6d2237) to head (e42db22).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #411   +/-   ##
=======================================
  Coverage    81.9%    81.9%           
=======================================
  Files         840      840           
  Lines      228058   228070   +12     
=======================================
+ Hits       186795   186820   +25     
+ Misses      41263    41250   -13