[CAP-65] Reusable module cache

[tomerweller]

This is a discussion thread for CAP-65 - Reusable module cache (by @graydon)

[graydon]

We've got some preliminary cost-improvement numbers from running the implementation of this side-by-side with current code on mainnet transactions: the charged (virtual) CPU costs seem to come down by an average of about 50%, with quite a bit of noise (some 10%, some 80%). Wall clock nanoseconds are a bit closer to the low end of improvement since CPU charges for module parsing were an overcharge, not a tight fit, but the improvement is still double-digit percentages.

[tomerweller]

Awesome work @graydon. Small nit on the title: I don't think that it's immediately obvious that "Module" in this context is in fact "WebAssembly Module"