wasmi should be removed as sc-executor backend. It was quite useful in the early days, but it will never be as fast as wasmtime which is currently the de-facto standard in Substrate. For the beginning it should probably be enough to just remove the crate and keep the current structure with sc-executor/sc-executor-wasmtime. In the future we may could merge these both crates as well to remove unneeded code and to simplify the entire structure.