Assert by level

[graydon]

Given the current (and potentially permanent) absence of typestate, enabling and disabling assertions by severity is a more reasonable requirement. Some assertions make more sense when in testing than in production. We should support (when we switch assertions to macros / library calls) passing a "level" parameter the same way the logging system can be parameterized by level.

[pcwalton]

I don't think this is necessarily backwards incompatible. Given that assertions are now macros, we have a lot of flexibility to beef up the syntax in backwards-compatible ways. Nominating for some other milestone.

[brson]

note that logging is no longer parameterized by level - we have separate macros for each

[pnkfelix]

accepted for production ready

[emberian]

/cc @huonw, #7869

[pnkfelix]

Assigning P-low.

[thestinger]

I think this is too out-of-date so I've opened #12049. I think we only need to distinguish between upholding invariants and sanity checks.