vmbase: Move heap.rs out of pvmfw

The module isn't pvmfw-specific so move it to vmbase and re-use the now
centralized implementation in Rialto and vmbase_example. This will allow
fully wrapping heap-initialization in vmbase in a future commit.

Deduplicate the dependency on buddy_system_allocator.

Test: TH
Change-Id: If39f46d99a3721001b5784f782577ae0c2a4b89d
diff --git a/vmbase/src/memory/util.rs b/vmbase/src/memory/util.rs
index 48007f3..b9ef5c9 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 64KB memory in bytes.
+pub const SIZE_64KB: usize = 64 << 10;
 /// The size of a 128KB memory in bytes.
 pub const SIZE_128KB: usize = 128 << 10;
 /// The size of a 2MB memory in bytes.