Install .mli and .cmt files.
Add .merlin.
Migrate to cstubs.
Add Debian/Ubuntu depexts, as libsodium is now recent enough.
Restore compatibility with pre-4.02.
Bump libsodium version requirement.
Add -Wall -pedantic to C compiler flags.
Install .cmti files instead of .cmt (fixes a typo).
Use -pedantic -Wall -Werror.

As we do not ship any C source, this is not a problem, unlike
the usual usage of -Werror in release builds.
Merge pull request #8 from whitequark/master

Migrate to cstubs
Bind crypto_sign_detached functions