Separate saphe from satysfi

[gfngfn]

TODO:

• Saphe:
  • Write satysfi-envelope.yaml for libraries during $ saphe solve
  • Develop used_as
  • Modify math for font packages
  • Write satysfi-deps.yaml before building when $ saphe build is invoked
  • Support $ saphe build for packages
  • Support $ saphe build for documents
  • Support $ saphe test for packages
  • Refactor the solver about registry local names
  • Initialize the store root config when nonexistent
  • Support $ saphe init
  • Separate $ saphe update from $ saphe solve
  • Re-support Markdown inputs
  • Take used_as into consideration when building
• SATySFi:
  • Load satysfi-deps.yaml
  • Load satysfi-envelope.yaml
  • Support $ satysfi build document
  • Support $ satysfi build package
  • Support $ satysfi test package
• Other:
  • Migrate unit tests
  • Modify how to handle direct sums in YAML
  • Register fixed packages (other than md-ja)
  • Register md-ja
  • Migrate demo/
  • Migrate doc/
  • Migrate integration tests in tests/ other than md-ja
  • Migrate integration tests using md-ja
  • Modify the registry format by introducing per-package release configs
  • Allow semver strings to have a suffix starting with -
  • Abolish hyph/
  • Abolish unidata/
  • Make package releases able to depend on packages of other registries

Nested PRs:

• Introduce local packages to Saphe #429
• Change the package config format #430
• Separate ErrorReporting from frontend/main.ml #432
• Modify the registry format #433
• Support semver prerelease #434
• Separate eval_document_file to BuildDocument module #435
• Develop how to load files in packages and use it for handling hyphenation dictionaries #436
• Remove ~/.saphe/unidata/ and develop unidata package #437

Possible future improvements:

• Add target/ directory for hiding intermediate files (specifically, {doc.,}satysfi-deps.yaml and doc.satysfi-aux)
• Make registry: "default" omissible
• Make the system more robust (e.g. recovery from errors that happened during a prior execution)
• Cache-handling operations