change "harden" to share a WeakMap of already-frozen objects

Our current def() function currently does too much work: it doesn’t remember what’s been frozen already, so it will re-freeze things like Function.prototype every time. To fix this, deepFreeze() needs to be turned into a “Freezer” object that retains a WeakMap of everything it has ever frozen, def() should take a Freezer, and the def() exposed as a global should close over the Freezer and deliver it here. deepFreezePrimordials() should use that same Freezer

This isn’t a security issue, merely a performance one.

[erights]

Noting again here that we’re likely to change terminology from “deeply frozen” and such to “hardened”.

[erights]

No, because Object.isFrozen is not transitive. If def stopped at a frozen object, it could miss non-frozen objects that were reachable from named properties of that frozen object:

const bad = Object.freeze([{}]);
// At this point, we have a frozen array containing 
// a non-frozen object in its transitive API surface.
def(bad);

If the call to def succeeds, then the transitive API surface starting at bad must be frozen. But an isFrozen check would stop too early.