All Kawa-related downloads now use our existing VerifiedDownload task
class. This gives us Gradle-integrated progress reporting,
incremental build support, build caching, correct dependencies, etc.
Using Kawa to compile Scheme into bytecode now also has proper
dependency management, incremental build support, and build caching.
Same goes for bundling these compiled bytecode files into jar archives
for later use in regression tests.
Also, when downloading kawa-chess, grab a specific commit hash rather
than whatever is the most recent master commit. If this project
changes in the future, we don't want our tests to break unexpectedly.
Perhaps we'd want to pick up any new kawa-chess commits; perhaps not.
Either way, that should be a conscious decision rather than something
that can happen behind our backs.
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