Introduce distinct integer types

This PR does the following things:
- Give incompatible representations for integers with different semantics:
  - `usize` is used for natural numbers for the public API.
  - `Nat` is used internally for natural numbers (essentially a stable `usize`).
  - `Word` is used for sequences of bits representing words in flash.
  - `Position` is used for word positions in the virtual storage.
- Only use fixed size integers to preserve overflow behavior between targets.
- Use little-endian representation instead of native representation for `Word`.

Alternatives:
- Run tests and fuzzing on 32-bits architecture (or some compatibility mode).
  This approach would have better readability than the current solution (less
  conversions at public API). However it would require additional setup and
  might not be viable long-term by restricting machines on which fuzzing is
  possible.
- Accept the behavior difference for tests and fuzzing. This approach would also
  have better readability. However checking for arithmetic overflow (and other
  `usize` related concerns like memory size) is more important.

libraries/persistent_store/src/storage.rs

@@ -37,9 +37,13 @@ pub type StorageResult<T> = Result<T, StorageError>;
 /// Abstracts a flash storage.
 pub trait Storage {
     /// The size of a word in bytes.
+    ///
+    /// A word is the smallest unit of writable flash.
     fn word_size(&self) -> usize;
 
     /// The size of a page in bytes.
+    ///
+    /// A page is the smallest unit of erasable flash.
     fn page_size(&self) -> usize;
 
     /// The number of pages in the storage.