vec::with_capacity: do one alloc for non-managed + ptr module improvements

[thestinger]

f74250e r=huonw

externmod extra;use std::vec;#[bench]fnbench_with_capacity(b:&mut extra::test::BenchHarness){
do b.iter{let _v: ~[u8] = vec::with_capacity(1024);}}

Before:

test bench_with_capacity ... bench: 293 ns/iter (+/- 0)

After:

test bench_with_capacity ... bench: 125 ns/iter (+/- 3)

[thestinger]

Tests all passed, bors is just being weird.