std: Move rand to librand.

This functionality is not super-core and so doesn't need to be included
in std. It's possible that std may need rand (it does a little bit now,
for io::test) in which case the functionality required could be moved to
a secret hidden module and reexposed by librand.

Unfortunately, using #[deprecated] here is hard: there's too much to
mock to make it feasible, since we have to ensure that programs still
typecheck to reach the linting phase.

src/etc/ziggurat_tables.py

@@ -11,7 +11,7 @@
 # except according to those terms.
 
 # This creates the tables used for distributions implemented using the
-# ziggurat algorithm in `std::rand::distributions;`. They are
+# ziggurat algorithm in `rand::distributions;`. They are
 # (basically) the tables as used in the ZIGNOR variant (Doornik 2005).
 # They are changed rarely, so the generated file should be checked in
 # to git.