Use visitor to extract memory usage information

[MichaReiser]

I'm not sure this is the right approach. It does require a fair amount of boilerplate code by any application that wants to extract memory usage information. However, it does allow applications to report detailed reports (e.g., aggregate by struct in addition to by memo) or even call custom aggregation functionality.

It also avoids the double visiting of the current implementation

I'm curious for your feedback

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[codspeed-hq]

CodSpeed Performance Report

Merging #944 will degrade performances by 4.52%

_{Comparing MichaReiser:memory-visitor (d22e36a) with master (53cd6b1)}

Summary

⚡ 2 improvements
❌ 2 regressions
✅ 8 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	amortized[Input]	2.5 µs	2.6 µs	-4.52%
⚡	amortized[InternedInput]	2.6 µs	2.4 µs	+6.06%
❌	amortized[SupertypeInput]	3.1 µs	3.2 µs	-4.5%
⚡	new[InternedInput]	4.5 µs	4.2 µs	+5.52%