fix(kernel): update outdated implementation of MemoryBlock::next_ptr (#509)

`MemoryBlock::next_ptr` used to calculate the offset to the data of the
next block, but it should be returning the start of the `MemoryBlock`
structure in memory. Also updates the tests for this case.

This was discovered when trying to update the kernel in the go-sdk:
https://github.com/extism/go-sdk/pull/32

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: zshipko <zshipko@users.noreply.github.com>

kernel/src/lib.rs

@@ -283,7 +283,7 @@ impl MemoryRoot {
 
         // Bump the position by the size of the actual data + the size of the MemoryBlock structure
         self.position.fetch_add(
-            length + core::mem::size_of::<MemoryBlock>() as u64 - 1,
+            length + core::mem::size_of::<MemoryBlock>() as u64,
             Ordering::SeqCst,
         );
 
@@ -316,9 +316,7 @@ impl MemoryBlock {
     /// is calculated based on metadata provided by the current block
     #[inline]
     pub unsafe fn next_ptr(&mut self) -> *mut MemoryBlock {
-        self.data
-            .as_mut_ptr()
-            .add(self.size + core::mem::size_of::<MemoryBlock>()) as *mut MemoryBlock
+        self.data.as_mut_ptr().add(self.size) as *mut MemoryBlock
     }
 
     /// Mark a block as free