Replace `structopt` with `clap`

[Expyron]

[Stranger6667]

What is the reason for this change?

[Expyron]

Structopt is not maintained anymore: https://github.com/TeXitoi/structopt/issues/525 Its author recommends moving to clap v3.

[codecov[bot]]

Codecov Report

Merging #384 (84ed9c9) into master (5da8685) will increase coverage by 0.01%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
+ Coverage   82.50%   82.52%   +0.01%     
==========================================
  Files          57       57              
  Lines        5562     5579      +17     
==========================================
+ Hits         4589     4604      +15     
- Misses        973      975       +2     

Impacted Files	Coverage Δ
jsonschema/src/main.rs	0.00% <ø> (ø)
jsonschema/src/keywords/format.rs	88.63% <78.57%> (-0.61%)	:arrow_down:
jsonschema/src/compilation/options.rs	87.34% <100.00%> (+0.21%)	:arrow_up:
jsonschema/src/error.rs	64.14% <0.00%> (ø)
jsonschema/src/lib.rs	82.43% <0.00%> (+1.35%)	:arrow_up:

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[Julian]

As is I think this also is triggering GitHub security warnings for users of jsonschema-rs -- e.g. on bowtie, due to the transitive dependency on ansi_term, I see:

EDIT: It seems the advisory was now retracted, so the above isn't true anymore.

[Stranger6667]

Sorry for the delay! Merged :) I'll make a new release soon