Function make_handler 

pub fn make_handler<'a>(
    counter: &'a mut u64,
) -> impl FnMut(&mut Execution, &[u8], u64, u64, u64, u64, u64, u64) -> Result<u64, ()> + 'a

The standard syscall handler for the test fixture.

Captures counter from the caller; memory access goes through the &mut Execution passed on each invocation.