Fail build on non 64-bit targets

[Ablu]

As discussed on Slack and during sync meetings, we should make it explicit that we only support 64bit. While other architectures may be viable to support, currently we assume that usize == u64 and nobody expressed interest in 32bit support.

Closes: #225

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[roypat]

Nice, thanks for picking this up!

[Ablu]

rebased