Improve Vec::resize so that it can be used in Read::read_to_end

We needed a more efficient way to zerofill the vector in read_to_end.
This to reduce the memory intialization overhead to a minimum.

Use the implementation of `std::vec::from_elem` (used for the vec![]
macro) for Vec::resize as well. For simple element types like u8, this
compiles to memset, so it makes Vec::resize much more efficient.

src/libstd/lib.rs

@@ -146,6 +146,7 @@
 #![feature(unique)]
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(vec_push_all)]
+#![feature(vec_resize)]
 #![feature(wrapping)]
 #![feature(zero_one)]
 #![cfg_attr(windows, feature(str_utf16))]