Refactor implementation of `Decode::skip` and `Decode::encoded_fixed_size` #270

Job	Run time
set-image	1s
fmt	1m 4s
clippy	1m 14s
bench-rust-nightly (bit-vec)	17m 29s
checks	1m 21s
bench-rust-nightly (bytes)	16m 22s
miri (bit-vec)	3h 55m 30s
tests	2m 1s
bench-rust-nightly (generic-array)	16m 25s
miri (bytes)	3m 46s
miri (generic-array)	3m 49s
miri (arbitrary)	3m 30s
bench-rust-nightly (derive)	16m 27s
bench-rust-nightly (max-encoded-len)	16m 27s
build-linux-ubuntu-amd64	1m 10s
publish-dry-run	1m 13s
	5h 37m 49s

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