refactor(encoding): remove `as` casts #975

Job	Run time
Check	1m 37s
Test Suite	1m 54s
Rustfmt	11s
Clippy	1m 15s
Check rustdoc intra-doc links	30s
Cross compile (wasm32-unknown-unknown)	39s
Cross compile (powerpc64-unknown-linux-gnu)	19s
Cross compile (powerpc-unknown-linux-gnu)	1m 12s
Cross compile (armv7-unknown-linux-gnueabihf)	19s
	7m 56s

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