Move #[global_allocator] into allocator module (#714)

The Rust issue that the #[global_allocator] cannot be defined in submodules was fixed.

src/lib.rs

@@ -9,7 +9,6 @@
 extern crate alloc;
 
 use core::panic::PanicInfo;
-use linked_list_allocator::LockedHeap;
 
 pub mod allocator;
 pub mod gdt;
@@ -18,9 +17,6 @@ pub mod memory;
 pub mod serial;
 pub mod vga_buffer;
 
-#[global_allocator]
-static ALLOCATOR: LockedHeap = LockedHeap::empty();
-
 pub fn init() {
     gdt::init();
     interrupts::init_idt();