Split out and organise openapi code

[arlyon]

This is a first attempt into breaking up the openapi code and making it more approachable. The main focus is on top-level visibility, simplifying the APIs used in main.

Usage is as follows:

• load the spec
• create a Metadata
• create a UrlFinder
• call Metadata::get_files to get FileGenerators for each object in the API
• call FileGenerator::write for each file
• call FileGenerator::write for the generators produced by the first write

I also took the opportunity to:

• replace prints with proper logging
• fix a number of clippy lints
• selectively enable / disable Boxing based on a (currently primitive) heuristic

This will hopefully provide a base to better define the abstractions now that the code is a little tidier.

[codecov]

Codecov Report

Merging #160 (570a30a) into master (af95ea4) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master    #160      +/-   ##
=========================================
- Coverage    4.43%   4.41%   -0.02%     
=========================================
  Files         119     120       +1     
  Lines       10102   10187      +85     
=========================================
+ Hits          448     450       +2     
- Misses       9654    9737      +83     

Impacted Files	Coverage Δ
openapi/src/file_generator.rs	0.00% <0.00%> (ø)
openapi/src/metadata.rs	0.00% <0.00%> (ø)
src/resources/generated/account.rs	1.86% <ø> (ø)
src/resources/generated/alipay_account.rs	0.00% <ø> (ø)
src/resources/generated/api_errors.rs	0.00% <ø> (ø)
src/resources/generated/application.rs	0.00% <ø> (ø)
src/resources/generated/application_fee.rs	0.00% <ø> (ø)
src/resources/generated/balance.rs	0.00% <ø> (ø)
src/resources/generated/balance_transaction.rs	0.00% <ø> (ø)
src/resources/generated/bank_account.rs	0.00% <ø> (ø)
... and 65 more

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[arlyon]

CI is currently broken due to an MSRV bump issue. More context: tokio-rs/tracing#1913