refactor binary interface types from usize to their specific sizes

[FreddieRidell]

Feature Request

Summary

In some places usize is being used in situations where it would be better to explicitly define what bit size we want to use. One example can be found here:

hypercore/src/storage/node.rs

Lines 53 to 54 in c7c8757

	// TODO: This will blow up on 32 bit systems, because usize can be 32 bits.
	let length = reader.read_u64::<BigEndian>()? as usize;

Motivation

The crate is currently built with a 64 bit word size in mind, this will cause issues when run on a 32-bit system

Explanation

Review all uses of usize, evaluate if u64 would be more appropriate, change the types, fix any compiler errors that occour. (optional: build and run on a 32 bit system to check compatability)

Unresolved Questions

Is this something that should be done now? are there valid reasons to postpone this work? Are there any other relevant issues that have not been described in this ticket

[yoshuawuyts]

@FreddieRidell I fully agree with you here, and would welcome patches to change this. We've done this in a few other crates too already. There shouldn't be any blockers to landing such changes.

[FreddieRidell]

I've put up a PR, I had a look through the codebase and this was the only place where we obviously should use u64 over usize.

Unfortunately it seems there's no way to easily test 32-bit on travis-ci, and I don't have any way to test it available.