Fix modulus leading zeros calculation for (Boxed)`MontyForm` #1916

Job	Run time
build (1.83.0, thumbv7em-none-eabi)	44s
build (1.83.0, wasm32-unknown-unknown)	40s
build (stable, thumbv7em-none-eabi)	38s
careful	1m 21s
clippy	11s
build (stable, wasm32-unknown-unknown)	42s
build-benchmarks	40s
cross (aarch64-unknown-linux-gnu, stable)	2m 40s
minimal-versions	27s
cross (powerpc-unknown-linux-gnu, stable)	2m 55s
doc	30s
miri (x86_64-unknown-linux-gnu)	4m 15s
miri (s390x-unknown-linux-gnu)	4m 18s
rustfmt	8s
test (i686-unknown-linux-gnu, 1.83.0, sudo apt update && sudo apt install gcc-multilib)	1m 51s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 42s
test (x86_64-unknown-linux-gnu, 1.83.0)	1m 35s
test (x86_64-unknown-linux-gnu, stable)	1m 29s
	26m 46s

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