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/lib.rs b/vmbase/src/lib.rs
index 7fc7b20..88bad8b 100644
--- a/vmbase/src/lib.rs
+++ b/vmbase/src/lib.rs
@@ -23,6 +23,7 @@
 pub mod console;
 mod entry;
 pub mod fdt;
+pub mod heap;
 mod hvc;
 pub mod layout;
 mod linker;