feat: Use quickcheck to test allocations, fix one bug that was uncovered. (#662)

- Adds quickcheck tests for alloc/free/load/store from the kernel, I
wasn't able to include these in the new wasm-bindgen tests because
`getrandom` isn't available on wasm32-unknown-unknown
- Fixes a bug in the calculation of how much memory is needed to
allocate the next block in the kernel. This bug is triggered when
allocating at the end of a page, the size of the MemoryBlock value
wasn't being taken in consideration when determining whether or not to
call memory.grow

kernel/src/lib.rs

@@ -270,12 +270,13 @@ impl MemoryRoot {
 
         // Get the number of bytes available
         let mem_left = self_length - self_position - core::mem::size_of::<MemoryRoot>() as u64;
+        let length_with_block = length + core::mem::size_of::<MemoryBlock>() as u64;
 
         // When the allocation is larger than the number of bytes available
         // we will need to try to grow the memory
-        if length >= mem_left {
+        if length_with_block >= mem_left {
             // Calculate the number of pages needed to cover the remaining bytes
-            let npages = num_pages(length - mem_left);
+            let npages = num_pages(length_with_block - mem_left);
             let x = core::arch::wasm32::memory_grow(0, npages);
             if x == usize::MAX {
                 return None;