Move from usize to u64 backing storage for Vobs.

[ltratt]

By using variable sized storage, we couldn't serialise a Vob on a 64-bit machine and deserialise on a 32-bit machine (i.e. we couldn't cross-compile grmtools to WASM). This relatively simple change makes that possible (since the backing is now always u64 no matter the machine). [See https://github.com/softdevteam/sparsevec/pull/21 which made this PR possible.]

This may stop people asking me when grmtools will support wasm (it always "has" in the sense that if you compiled it on a 32-bit machine it worked. But, in fairness, I don't have access to a 32-bit development environment any more!

[ratmice]

looks pretty straight forward to me, do we need to rebase on master, or does bors manage to take care of that?

[ltratt]

It'll create a merge commit (so if it can't merge it safely, it'll bork in one way another).

[ratmice]

bors r+

[bors[bot]]

Build succeeded:

• buildbot/buildbot-build-script