Simplify `Composite::evaluate()` final value construction. #258

Job	Run time
no_std	27s
miri	1m 34s
build (ubuntu, stable, locked, true)	12m 50s
build (windows, stable, locked, false)	22m 26s
build (macos, stable, locked, false)	22m 0s
build (ubuntu, nightly, locked, false)	20m 2s
build (ubuntu, beta, locked, false)	20m 57s
build (ubuntu, stable, latest, false)	13m 34s
build (ubuntu, stable, minimal, false)	12m 50s
deploy	8s
features	26m 48s
fuzz	15m 20s
	2h 48m 56s

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