This commit extracts out server code into src/Server.zig and uses it
both in the main CLI as well as `zig objcopy`.

std.Build.ObjCopyStep now adds `--listen=-` to the CLI for `zig objcopy`
and observes the protocol for progress and other kinds of integrations.

This fixes the last two test failures of this branch when I run
`zig build test` locally.