.init_array support and opt-in heap allocator

[japaric]

With .init_array support, external crates (dependencies) can run code before
main is executed.

Usage looks like this:

extern crate alloc_cortex_m;
#[macro_use]  // for `init_array!`
extern crate r0;

init_array!(alloc, {
    alloc_cortex_m::init(..);
});

However, the implementation is not reliable because there's no mechanism to
force rustc to emit symbols when they are apparently not used. Something like
GCC's __attribute(used)__ would fix this. See rust-lang/rfcs#1002 and
rust-lang/rfcs#1459 for previous request of this feature.

[toothbrush7777777]

IIRC, specifying the item as pub will keep the symbol whether used or not.

[japaric]

@toothbrush7777777 Yes, but with that approach the symbols end up being part of your public API.

[japaric]

This template has been deprecated in favor of the cortex-m-quickstart template.