Update Allocator trait implementation to compile on nightly

[rusty122]

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Fixes issue #94

The wee_alloc build has been broken for a bit because the Rust Allocator (previously called Alloc) trait has changed interfaces. This PR updates the wee_alloc implementation to compile and run on nightly.

There are probably some more cleanup possible and different testing possibilities with the new interface, but I tried to make the minimal changes necessary to fix the build and unblock other contributions.

Some functions and methods that are still experimental I implemented as helper-functions to avoid requiring nightly for the library, but left TODO comments to update once they stabilize.

Haven't made a ton of Rust contributions so let me know if I missed anything here!

[rusty122]

cc @pepyakin @DrGoldfire @ZackPierce if anyone can review