Macro default_allocator 

macro_rules! default_allocator {
    () => { ... };
}

Install the default bump allocator over the SVM heap region.

Opt-in counterpart to no_allocator!: use this when a program needs alloc (e.g. heap Vec/String on a cold path) while keeping the zero-copy hot path allocation-free. Never frees within an instruction; the whole heap is reclaimed when the instruction returns.