Handle sparse storage properly

[khernyo]

The current implementation doesn't seem to do sparse allocation like https://github.com/random-access-storage/random-access-memory.

In short: RandomAccessMemory::buffers should be of type Vec<Option<Vec<u8>>> instead of the current type Vec<Vec<u8>>.

I can see two problems with the current implementation:

1. RandomAccessMemory::write() allocates all pages before page_num unnecessarily. See

   random-access-memory/src/lib.rs

   Line 86 in 957856c

   self.buffers.resize(page_num + 1, buf);

2. RandomAccessMemory::del() seems to be broken. When removing a page it shifts all following pages thereby corrupting the stored data. Compare

   random-access-memory/src/lib.rs

   Line 183 in 957856c

   self.buffers.remove(i);

   to https://github.com/random-access-storage/random-access-memory/blob/da2d94570fc48674c22746b288bf65c3bf44015d/index.js#L93

[bltavares]

Should this adopt another of project on datrs that already seems to handle sparse data using Vec<Option<Page>>?

https://github.com/datrs/memory-pager/blob/8f3f650af47a0044e56b48d6aeb1037a1a0555c5/src/lib.rs#L24

[khernyo]

Seems like a good idea. I'm not overly familiar with that project, though.