ecdsa: reduce `z` mod `q` when performing RFC6979 (#777) #52

Job	Run time
test (1.71.0)	18s
build (thumbv7em-none-eabi, 1.71.0)	19s
test (stable)	19s
build (thumbv7em-none-eabi, stable)	39s
build (wasm32-unknown-unknown, 1.71.0)	21s
build (wasm32-unknown-unknown, stable)	18s
	2m 14s

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