Convert extra::ebml to extra::atom_trees

[brson]

This module doesn't actually implement ebml correctly and only exists to support rustc's metadata. Make a few changes to optimize it:

• Rename it from ebml to atom_trees, change all internal naming
• Make tags an atom (fourcc tag) of native endianness + 32-bit length with native endianness. This should be faster than the variable-length tags ebml uses.

cc #9255

[emberian]

Please also use little endian, not big endian.

[emberian]

(Oh, it seems that was mentioned; nevermind)

[michaelwoerister]

What's an atom tree? I can't find anything with that name on the internet.

[emberian]

cc @pcwalton (I recall him talking about this).

I think it's just whatever ebml is now, though... not really sure.

[ahmedcharles]

Is this done or still being worked on?

[huonw]

Neither, if you wish to take it, it's all yours.