Consider supporting diagnose_if for preconditions

[nemequ]

Clang has a rather interesting diagnose_if attribute. I think it would cause a lot of code duplication with assertions, and be compile-time only, but it may also help people catch stuff earlier. At some point hopefully static analyzers will pick up support for it; clang-analyzer may even support it already (I should check…)

[nemequ]

Available in v8