Custom `alloc`

[ojeda]

We will most likely need to customize alloc to ensure fallible allocations everywhere, as well as being able to pass flags etc.

[ojeda]

From @Ericson2314 in #204:

i have opened rust-lang/wg-allocators#87 and rust-lang/rust#84266 about this. Should be easy to pass the appropriate flags to rustc to disable that feature if the PR or something like it lands.

[Ericson2314]

rust-lang/rust#84266 is now merged. The raw CFG is hard to use from Cargo but should be fine for us. This will remove all the problematic functionality at compile time.