Add missing implementation of from_ast for Record

[yannham]

While testing #2083 I realized that I left a todo!() in the translation of the new AST to mainline Term. This PR adds the missing case. I also removed RecordAttrs that were copy pasted when introducing the new AST, but since we only store an open flag, I ended up using a simple bool field so this type is just unused now.

[github-actions]

Bencher Report

Branch	rfc007/add-missing-from-record-ast
Testbed	ubuntu-latest

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Benchmark	Latency	nanoseconds (ns)
fibonacci 10	📈 view plot 🚷 view threshold	489,290.00
foldl arrays 50	📈 view plot 🚷 view threshold	1,681,200.00
foldl arrays 500	📈 view plot 🚷 view threshold	6,762,100.00
foldr strings 50	📈 view plot 🚷 view threshold	7,139,600.00
foldr strings 500	📈 view plot 🚷 view threshold	63,429,000.00
generate normal 250	📈 view plot 🚷 view threshold	45,500,000.00
generate normal 50	📈 view plot 🚷 view threshold	2,159,700.00
generate normal unchecked 1000	📈 view plot 🚷 view threshold	3,369,900.00
generate normal unchecked 200	📈 view plot 🚷 view threshold	773,110.00
pidigits 100	📈 view plot 🚷 view threshold	3,250,800.00
pipe normal 20	📈 view plot 🚷 view threshold	1,487,900.00
pipe normal 200	📈 view plot 🚷 view threshold	10,416,000.00
product 30	📈 view plot 🚷 view threshold	827,630.00
scalar 10	📈 view plot 🚷 view threshold	1,506,500.00
sum 30	📈 view plot 🚷 view threshold	830,210.00

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