Explicitly build and install OCaml bytecode files

Bytecode is no longer built by default, but it’s still essential on
Debian platforms where OCaml has no native compiler. Run `make byte`
and `make install-byte` to install bytecode.

In the future, we may want to separate bytecode binaries and libraries
into their own packages so users on platforms with native support don’t
have to spend disk space on bytecode. For now, though, continue
bundling native code and bytecode into the same package.