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.
The src/tools directory contains the cretonne-tools crate which will build
binaries for testing cretonne.
The src/libctonfile directory contains the ctonfile library crate which
provides reading and writing of .cton files.
