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/pvmfw/src/main.rs b/pvmfw/src/main.rs
index 8c5d5d5..61af048 100644
--- a/pvmfw/src/main.rs
+++ b/pvmfw/src/main.rs
@@ -28,7 +28,6 @@
 mod exceptions;
 mod fdt;
 mod gpt;
-mod heap;
 mod helpers;
 mod instance;
 mod memory;
@@ -49,6 +48,7 @@
 use pvmfw_avb::Capability;
 use pvmfw_avb::DebugLevel;
 use pvmfw_embedded_key::PUBLIC_KEY;
+use vmbase::heap;
 use vmbase::memory::flush;
 use vmbase::memory::MEMORY;
 use vmbase::virtio::pci;