Implement core::str::from_cstr_len with a test, close issue #1666

[uasi]

No description provided.

[brson]

Thanks! If you want, this function could be implemented more efficiently with sys::memcpy. Something vaguely like

let buf: [u8] = [];
vec::reserve(buf, len + 1);
vec::as_buf(buf) {|buf| sys::memcpy(buf, sbuf, len) };
vec::unsafe::set_len(buf, len);
buf += [0u8];
let s = unsafe::reinterpret_cast(buf);
unsafe::leak(buf);
ret s;

This is two allocations (the initial [] and the call to reserve) and a memcpy. In the current implementation the vector will be reallocated each time its size hits a power of two and from_bytes will copy the entire buffer again.

[brson]

That's ptr::memcpy, not sys

[uasi]

Okay, I'll give it a try. If it works I'll make another pull req.