Backport upstream changes to master - #6

* The coremidi dependencies was updated to pick up a more recent
 non-released branch. This was done so that we don't need to vendor old
 versions of the core-foundation and core-foundation-sys crates
* The wasm32 target was removed in order not to have to vendor all its
 dependencies (which would be unused)

We'll handle picking the right version on the gecko side.

This reduces the differences with upstream.

Cargo.toml updates

Upgrade to alsa 0.7

RustAudio/rust-jack#154 

Fixes Boddlnagg#94 

Previously, the ALSA backend created a new `Vec<pollfd>` and
immediately called `Vec::set_len`. This exposes uninitialized
`pollfd`s, which causes undefined behaviour. This commit fixes this by
initializing the `Vec` with invalid, but still initialized, `pollfd`s
before filling it. In addition, this commit simplifies the code
slightly by removing the usage of `Vec::into_boxed_slice`.
Fixes Boddlnagg#110.

- Replace memalloc crate dependency with rust std alloc/dealloc