Use manual base 64 encoding, file paths only.

[wlandau]

Summary

Fixes #630 and #572. Also related: #631. Significant speed improvement while preserving back compatibility with old projects. See #630 for discussion and benchmarks.

Related GitHub issues and pull requests

• Ref: storr::encode64() overhead #572, Pre-encode file paths internally #630, Pre-work for file encoding #631

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #633 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #633   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          82     82           
  Lines        6951   6968   +17     
=====================================
+ Hits         6951   6968   +17

Impacted Files	Coverage Δ
R/subspace.R	100% <ø> (ø)	⬆️
R/code_analysis_utils.R	100% <ø> (ø)	⬆️
R/handlers.R	100% <ø> (ø)	⬆️
R/strings.R	100% <100%> (ø)	⬆️
R/cache_ui.R	100% <100%> (ø)	⬆️
R/build_times.R	100% <100%> (ø)	⬆️
R/check.R	100% <100%> (ø)	⬆️
R/diagnose.R	100% <100%> (ø)	⬆️
R/outdated.R	100% <100%> (ø)	⬆️
R/encoding.R	100% <100%> (ø)
... and 17 more

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