libcore: unify gen_<type> methods on rand::RngUtil into the generic gen.

This moves all the basic random value generation into the Rand instances for
each type and then removes the `gen_int`, `gen_char` (etc) methods on RngUtil,
leaving only the generic `gen` and the more specialised methods.

Also, removes some imports that are redundant due to a `use core::prelude::*`
statement.

src/libstd/test.rs

@@ -19,20 +19,9 @@ use getopts;
 use sort;
 use term;
 
-use core::cmp::Eq;
-
 use core::to_str::ToStr;
-use core::either::Either;
-use core::either;
-use core::io::WriterUtil;
-use core::io;
 use core::comm::{stream, SharedChan};
-use core::option;
 use core::prelude::*;
-use core::result;
-use core::str;
-use core::task;
-use core::vec;
 
 pub mod rustrt {
     use core::libc::size_t;
@@ -608,12 +597,8 @@ pub mod bench {
     use time::precise_time_ns;
     use test::{BenchHarness, BenchSamples};
     use stats::Stats;
-
-    use core::num;
+    use core::prelude::*;
     use core::rand::RngUtil;
-    use core::rand;
-    use core::u64;
-    use core::vec;
 
     pub impl BenchHarness {