fix: remove dead halftone code + fix docstrings + switch to from_hf() #139

Job	Run time
format/stable	20s
native-backend/stable	37s
cross_test/s390x-unknown-linux-gnu/stable	1m 59s
clippy/stable	21s
cross_test/i686-unknown-linux-gnu/stable	1m 52s
Pass MSRV values to other jobs	2s
nostd/thumbv6m-none-eabi/stable	31s
docs/nightly	0s
miri	0s
cargo-careful	0s
blas-msrv	1m 2s
tests/1.64.0	1m 1s
tests/beta	1m 6s
tests/stable	57s
conclusion	0s
	9m 48s

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