auto merge of #15284 : apoelstra/rust/bitv-methods, r=cmr

The types `Bitv` and `BitvSet` are badly out of date. This PR:
- cleans up the code (primarily, simplifies `Bitv` and implements `BitvSet` in terms of `Bitv`)
- implements several new traits for `Bitv`
- adds new functionality to `Bitv` and `BitvSet`
- replaces internal iterators with external ones
- updates documentation
- minor bug fixes

This is a significantly souped-up version of PR #15139 and is the result of the discussion there.

src/test/run-pass/bitv-perf-test.rs

@@ -13,8 +13,8 @@
 use std::collections::Bitv;
 
 fn bitv_test() {
-    let mut v1 = box Bitv::new(31, false);
-    let v2 = box Bitv::new(31, true);
+    let mut v1 = box Bitv::with_capacity(31, false);
+    let v2 = box Bitv::with_capacity(31, true);
     v1.union(v2);
 }
 

src/test/run-pass/issue-11736.rs

@@ -16,7 +16,7 @@
 fn main() {
     // Generate sieve of Eratosthenes for n up to 1e6
     let n = 1000000u;
-    let sieve = Bitv::new(n+1, true);
+    let sieve = Bitv::with_capacity(n+1, true);
     let limit: uint = (n as f32).sqrt() as uint;
     for i in range(2, limit+1) {
         if sieve[i] {