CircleCI slimming [WIP]

[sbillig]

• only run b_ubu_ossfuzz during nightly fuzz run (~24m Large)
• only run chk_antlr_grammar when the grammar changes (~11m Small)
• use ccache for solc builds
• docs-only PRs should only run a build-docs github action (big savings for many PRs)
• skip archlinux build & test for PRs, unless a cmake file changes (~17m Large)
• only run tests on linux for most commits, and full ci on PR approval or manual 'run-full-ci' github label? (big savings)
• move ossfuzz nightly job to a "scheduled pipeline", and run it half as often? (24m Large, 67m Small)
• investigate b_alpine_docker (44m Medium)
  > # Always build from scratch to use current packages.
  > # This job is only meant to run nightly so build time is not an issue.
  > docker_layer_caching: false

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[sbillig]

help @r0qs @clonker ?