The integration tests use both libcretonne and libreader, so moving them avoids
the circular dev-dependency.
Also go back to building everything under src/tools/target to avoid rebuilding
the libraries when cargo is invoked in different subdirectories. This speeds up
test-all.sh quite a bit.
Finally, skip the pure debug build. We build "cargo test" and "cargo build
--release" which should cover everything we need.
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