Fix type inference in derive decode (#256)

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check-rust-stable-no_derive_no_std_full Test There has been a runner system failure, please try again
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test-rust-stable Test There has been a runner system failure, please try again
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test-rust-stable-no_derive Test There has been a runner system failure, please try again
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check-rust-stable-no_derive_no_std Test There has been a runner system failure, please try again
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bench-rust-nightly Test There has been a runner system failure, please try again
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check-rust-stable-no_derive_full Test There has been a runner system failure, please try again
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