defineProperty based property memoization

[airhorns]

This implements a better performing getter function for readonly instances. Before this, we used the same definition of the getter function for all views, which ended up megamorphic, because it accessed a very wide variety of properties for every instance! Instead, this evals a monomorphic getter for each one. I also changed the object that stores the memos to have a fixed shape from birth by evaling it out as well.

I also changed us to use one object to store all the memoized values, instead of two. We were previously using two in order to implement memoization of undefined and null correctly, but with the new strategy to initialize an object with slots for every memo from the start, we can populate it with a $notYetMemoized symbol that indicates if we have memoized or not.

[codspeed-hq]

CodSpeed Performance Report

Merging #92 will degrade performances by 27.67%

_{Comparing faster-get-5 (58da4e6) with main (f183291)}

Summary

⚡ 4 improvements
❌ 1 regressions
✅ 3 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	faster-get-5	Change
⚡	instantiating a large root	4 ms	3.5 ms	+14.55%
⚡	accessing memoized getter properties of a class model	15.2 µs	10.5 µs	+44.43%
⚡	accessing memoized null property of a class model	14.4 µs	11.2 µs	+28.64%
⚡	accessing unmemoized getter properties of a class model	66.6 µs	44.9 µs	+48.33%
❌	accessing unmemoized null property of a class model	17.5 µs	24.1 µs	-27.67%