Use once crate to ensure that memory::init is only called once

Cargo.toml

@@ -13,3 +13,4 @@ spin = "0.4.5"
 multiboot2 = "0.1.0"
 bitflags = "0.7.0"
 x86_64 = "0.1.2"
+once = "0.3.3"

src/lib.rs

@@ -18,6 +18,8 @@ extern crate multiboot2;
 #[macro_use]
 extern crate bitflags;
 extern crate x86_64;
+#[macro_use]
+extern crate once;
 
 #[macro_use]
 mod vga_buffer;

src/memory/mod.rs

@@ -11,6 +11,8 @@ pub const PAGE_SIZE: usize = 4096;
 
 
 pub fn init(boot_info: &BootInformation) {
+    assert_has_not_been_called!("memory::init must be called only once");
+
     let memory_map_tag = boot_info.memory_map_tag().expect(
         "Memory map tag required");
     let elf_sections_tag = boot_info.elf_sections_tag().expect(