pvmfw: Prepare heap.rs for move

Allow client code to control the size of the HEAP through a macro.

Make the SAFETY comment a compliant docstring.

Test: TH
Change-Id: I386e0c2e070e8c96f90560881005af827607fecc
diff --git a/vmbase/src/memory/util.rs b/vmbase/src/memory/util.rs
index 04d42cd..48007f3 100644
--- a/vmbase/src/memory/util.rs
+++ b/vmbase/src/memory/util.rs
@@ -22,6 +22,8 @@
 
 /// The size of a 4KB memory in bytes.
 pub const SIZE_4KB: usize = 4 << 10;
+/// The size of a 128KB memory in bytes.
+pub const SIZE_128KB: usize = 128 << 10;
 /// The size of a 2MB memory in bytes.
 pub const SIZE_2MB: usize = 2 << 20;
 /// The size of a 4MB memory in bytes.