PacketBuffer rewrite

[Boddlnagg]

This adresses the points discussed in #5. Details:

• Methods that add package data now take the data as &[u8] to not require the caller to allocate a Vec
• Creating a buffer with at most 28 bytes of data (minus the headers this corresponds to e.g. a single packet with 14 bytes of data), will not allocate on the heap, but store the buffer inline on the stack. The inline size could be increased at the cost of growing the PacketBuffer type.
• The implementation ensures correct 4-byte alignment of packets on ARM.
• The empty new() constructor has been removed, because having the buffer always contain at least one package allows some optimizations and I can't see reason for ever needing an empty PacketBuffer. new now has the signature of from_data instead.
• I renamed with_data to push_data, because I think of it more as similar to the Vec API instead of a builder API. push_data still returns &mut self, so it can be chained. If you want me to undo these API changes, I can do that.
• I renamed PacketList::length() to len() to make it consistent with Rust's standard library (all the methods to get the length of something are called len(), see https://static.rust-lang.org/doc/master/std/index.html?search=length)
• Multiple non-sysex packets with the same timestamp are now combined into a single packet by push_data. This matches the behavior of MIDIPacketListAdd, and there are unit tests to ensure this (by comparing the result of using MIDIPacketListAdd vs. PacketBuffer).

This PR also includes an upgrade to version 2.0 of coremidi-sys (the first two commits), getting rid of coremidi_sys_ext. If you want to have these as a seperate PR, I can do that, too.

[Boddlnagg]

The merge commit that you pushed introduced a failure. I force-pushed a rebased version with the fix (let's see if Travis agrees that it's correct 😉 ). And thanks for the review!

[Boddlnagg]

@chris-zen Thanks for the review! I will push a commit with the changes that you requested (is it okay if I just put them all in a single commit?). Actually, I am responsible for the 2.0 version of coremidi-sys, after looking at this library and talking to the developer of coremidi-sys 1.0, who happens to be a friend of mine 😉

[Boddlnagg]

I pushed a commit addressing the review comments

[Boddlnagg]

Thanks! Do you plan to release a new version soon? Then I will be able to update midir :-)

[chris-zen]

Done. Released 0.3.0