This migrates from zig 0.13 to 0.15.2. In addition to dealing with
breaking changes in the build system and standard library APIs, the
architecture was changed substantially. We now build a standalone
CLI, and use that to execute the commands. This avoids sandboxing issues
related to TLS and enables easier testing. The commit also includes a
simple zip implementation (store only, single file) which avoids the
platform restriction (i.e. this build can now theoretically work on
Windows).
