Macro default_allocator 

macro_rules! default_allocator {
    () => { ... };
    (heap = $len:expr) => { ... };
}

Install the default bump allocator over the SVM heap region. Opt-in counterpart to no_allocator! for programs that need alloc on a cold path. See hopper_native::BumpAllocator.

default_allocator!(heap = 128 * 1024) declares a larger heap, up to 256 KiB, for transactions that carry RequestHeapFrame.