Macro bump_allocator 

macro_rules! bump_allocator {
    ($bytes:expr) => { ... };
}

Install a BumpAlloc of $bytes as the program’s global allocator, and define reset_heap() beside it.

A macro rather than a static in this crate because #[global_allocator] must be unique per binary: a static here would force the arena on every program that links nub-rt, including the ones that never allocate.

nub_rt::bump_allocator!(64 * 1024);

#[nub_rt::endpoint(0)]
fn run(_: u64) -> u64 {
    reset_heap();          // re-entrant: safe to invoke repeatedly
    my_kernel() as u64
}

On host targets it expands to a reset_heap() that does nothing, so the same source builds both ways.