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.

Author: huonw

src/libcore/hashmap.rs

@@ -56,7 +56,7 @@ fn resize_at(capacity: uint) -> uint {
 pub fn linear_map_with_capacity<K:Eq + Hash,V>(
     initial_capacity: uint) -> HashMap<K, V> {
     let r = rand::task_rng();
-    linear_map_with_capacity_and_keys((*r).gen_u64(), (*r).gen_u64(),
+    linear_map_with_capacity_and_keys(r.gen(), r.gen(),
                                       initial_capacity)
 }