fix: do not prepare/release NonMoving space in nursery GC

[playX18]

Resolves #1560

[qinsoon]

I think modifying CommonGenPlan::prepare/release would be cleaner.

Have something like

    /// Prepare Gen. This should be called by a single thread in GC prepare work.
    pub fn prepare(&mut self, tls: VMWorkerThread) {
        let full_heap = !self.is_current_gc_nursery();
        if full_heap {
            self.full_heap_gc_count.lock().unwrap().inc();
            self.common.prepare(tls, full_heap);
        } else {
            self.los.prepare(full_heap);
        }
        self.nursery.prepare(true);
        self.nursery
            .set_copy_for_sft_trace(Some(CopySemantics::PromoteToMature));
    }

    /// Release Gen. This should be called by a single thread in GC release work.
    pub fn release(&mut self, tls: VMWorkerThread) {
        let full_heap = !self.is_current_gc_nursery();
        if full_heap {
            self.common.release(tls, full_heap);
         } else {
            self.common.los.release(tls, full_heap);
         }
        self.nursery.release();
    }

This would be doing the same thing as sticky Immix -- see how sticky Immix only prepares/releases LOS in a nursery GC and does not call common.prepare/release.

mmtk-core/src/plan/sticky/immix/global.rs

Lines 119 to 156 in fb806c0

	fn prepare(&mut self, tls: crate::util::VMWorkerThread) {
	if self.is_current_gc_nursery() {
	// Prepare both large object space and immix space
	self.immix.immix_space.prepare(
	false,
	Some(StatsForDefrag::new(self)),
	// We don't do anything special to unlog bits during nursery GC
	// because ProcessModBuf will set the unlog bits back.
	UnlogBitsOperation::NoOp,
	);
	self.immix.common.los.prepare(false);
	} else {
	self.full_heap_gc_count.lock().unwrap().inc();
	self.immix.prepare_inner(
	tls,
	// We will reconstruct unlog bits during tracing.
	UnlogBitsOperation::BulkClear,
	);
	}
	}
	fn release(&mut self, tls: crate::util::VMWorkerThread) {
	if self.is_current_gc_nursery() {
	self.immix.immix_space.release(
	false,
	// We don't do anything special to unlog bits during nursery GC
	// because ProcessModBuf has set the unlog bits back.
	UnlogBitsOperation::NoOp,
	);
	self.immix.common.los.release(false);
	} else {
	self.immix.release_inner(
	tls,
	// We reconstructred unlog bits during tracing. Keep them.
	UnlogBitsOperation::NoOp,
	);
	}
	}

[qinsoon]

The style checks for stable Rust failed, but they are irrelevant for this PR. No need to fix them here.

[qinsoon]

LGTM. Thanks.