Allocator update for use with GlobalAlloc feature

[willscott]

The rust Alloc trait is getting updated, per rust-lang/rust#49668

In particular, the u8 return types for methods like alloc are getting replaced by a new type *mut Opaque. The code in dlmalloc/mod.rs doesn't compile against this new interface, with error

error[E0053]: method `dealloc` has an incompatible type for trait
  --> libsip/src/dlmalloc/mod.rs:69:5
   |
69 |     unsafe fn dealloc(&mut self, ptr: ptr::NonNull<u8>, layout: Layout) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected extern type `std::alloc::Opaque`, found u8
   |
   = note: expected type `unsafe fn(&mut dlmalloc::Dlmalloc, std::ptr::NonNull<std::alloc::Opaque>, std::alloc::Layout)`
              found type `unsafe fn(&mut dlmalloc::Dlmalloc, std::ptr::NonNull<u8>, std::alloc::Layout)

Context: The use of global_allocator triggers rust to ask for the global_allocator feature to be added to the crate in current nightlies, which cascades to changing the Alloc trait.

[lachlansneff]

Oh no, not again! It's probably a good idea to get rid of the global allocator stuff in this crate and let rust handle it.

[willscott]

This was caused because of an out-of-date rust compiler version, not a libsip problem