Refactor implementation of `Decode::skip` and `Decode::encoded_fixed_size` #271

Job	Run time
set-image	1s
checks	1m 26s
clippy	1m 31s
bench-rust-nightly (bit-vec)	18m 12s
fmt	1m 0s
bench-rust-nightly (bytes)	17m 12s
miri (bit-vec)	3h 57m 43s
tests	3m 38s
bench-rust-nightly (generic-array)	17m 12s
miri (bytes)	3m 45s
bench-rust-nightly (derive)	17m 11s
miri (generic-array)	3m 44s
bench-rust-nightly (max-encoded-len)	17m 26s
miri (arbitrary)	3m 46s
build-linux-ubuntu-amd64	1m 19s
publish-dry-run	1m 15s
	5h 46m 21s

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