Convert mem::zeroed() / 0 to MaybeUninit::uninit() (#798)

* Convert `mem::zeroed()` / `0` to `MaybeUninit::uninit()`

As noted in #792 changes like this might help us more strictly identify
and validate that the argument in question is only relevant as an output
argument (i.e. structs with `sType` are read by the function call, even
if the caller strictly expects return values there, so that it can fill
in multiple structures in a `pNext` chain, and must hence be `Default`-
initialized).

* Use uninit `Vec`s with late `set_len()` call instead of zero-initialization

* Use `uninit()` instead of `-1` for file descriptors

* Introduce `set_vec_len_on_success()` helper for `Vec::with_capacity()`

ash-examples/src/bin/texture.rs

@@ -768,7 +768,6 @@ fn main() {
                     device.cmd_end_render_pass(draw_command_buffer);
                 },
             );
-            //let mut present_info_err = mem::zeroed();
             let present_info = vk::PresentInfoKHR {
                 wait_semaphore_count: 1,
                 p_wait_semaphores: &base.rendering_complete_semaphore,

ash-examples/src/bin/triangle.rs

@@ -423,7 +423,6 @@ fn main() {
                     device.cmd_end_render_pass(draw_command_buffer);
                 },
             );
-            //let mut present_info_err = mem::zeroed();
             let wait_semaphors = [base.rendering_complete_semaphore];
             let swapchains = [base.swapchain];
             let image_indices = [present_index];