std::vec: Fix hazards with uint overflows in unsafe code

Issue #8742

Add the method `.reserve_additional(n: uint)`: Check for overflow in
self.len() + n, and reserve that many elements (rounded up to next power
of two). Does nothing if self.len() + n < self.capacity() already.
std::vec: Add function vec::bytes::push_bytes

`push_bytes` is implemented with `ptr::copy_memory` here since this
function is intended to be used to implement `.push_str()` for str, so
we want to avoid the overhead.
std::str: Add bench tests for StrVector::connect() and for str::push_str
std::str: Fix overflow problems in unsafe code

See issue #8742
rt::io: Use vec::reserve_additional
std::at_vec: Fix segfault on overflow when resizing ~[@T]

Easy to reproduce:

    let mut v = ~[@1];
    v.resize(-1);  // success a.k.a silent failure
    v.push(@2); // segfault