Fix false negative in contract equality

[jneem]

This fixes a performance regression caused by a false negative in contract equality: a ValueContentRef::Term(Term::Var(_)) was comparing false against a ValueContentRef::CustomContract, instead of resolving the variable in its environment and comparing again.

[github-actions]

Bencher Report

Branch	contract-equality-var
Testbed	ubuntu-latest

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Benchmark	Latency	microseconds (µs)
diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	8,048.10 µs
diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	169,760.00 µs
diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	231.22 µs
diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	374.19 µs
fibonacci 10	📈 view plot 🚷 view threshold	194.36 µs
foldl arrays 50	📈 view plot 🚷 view threshold	394.32 µs
foldl arrays 500	📈 view plot 🚷 view threshold	2,889.20 µs
foldr strings 50	📈 view plot 🚷 view threshold	2,695.70 µs
foldr strings 500	📈 view plot 🚷 view threshold	27,019.00 µs
generate normal 250	📈 view plot 🚷 view threshold	30,875.00 µs
generate normal 50	📈 view plot 🚷 view threshold	1,406.20 µs
generate normal unchecked 1000	📈 view plot 🚷 view threshold	1,977.60 µs
generate normal unchecked 200	📈 view plot 🚷 view threshold	394.06 µs
init-diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	45,925.00 µs
init-diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	181,670.00 µs
init-diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	41,891.00 µs
init-diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	42,200.00 µs
pidigits 100	📈 view plot 🚷 view threshold	1,997.50 µs
pipe normal 20	📈 view plot 🚷 view threshold	586.67 µs
pipe normal 200	📈 view plot 🚷 view threshold	4,495.00 µs
product 30	📈 view plot 🚷 view threshold	248.46 µs
requests-benches/inputs/goto-perf.ncl-000	📈 view plot 🚷 view threshold	2,921.70 µs
requests-benches/inputs/large-record-tree.ncl-000	📈 view plot 🚷 view threshold	459,940.00 µs
requests-benches/inputs/large-record-tree.ncl-001	📈 view plot 🚷 view threshold	81.49 µs
scalar 10	📈 view plot 🚷 view threshold	469.49 µs
sum 30	📈 view plot 🚷 view threshold	249.38 µs

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