fiat-constify: filter out `let mut` declarations (#1057) #1637

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	42s
build (1.65.0, wasm32-unknown-unknown)	35s
build (stable, thumbv7em-none-eabi)	52s
test (1.65.0)	43s
build (stable, wasm32-unknown-unknown)	17s
test (stable)	9s
	3m 18s

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