Marked reserve function as unsafe

[kitcatier]

stdweb/src/webcore/global_arena.rs

Lines 69 to 76 in 9b418d9

	pub fn reserve< 'a, T >( length: usize ) -> RelativeSlice< 'a, T > {
	unsafe {
	let offset = reserve_impl( length * mem::size_of::< T >(), mem::align_of::< T >() );
	debug_assert_eq!( ARENA.memory.offset( offset as isize ) as usize % mem::align_of::< T >(), 0 );
	RelativeSlice { offset, length, tail: offset, phantom: PhantomData }
	}
	}

hello, if a function's entire body is unsafe, the function is itself unsafe and should be marked appropriately. Marking them unsafe also means that callers must make sure they know what they're doing.