Make WasmtimeEngine a marker struct

[jprendes]

This PR makes the WasmtimeEngine a marker struct which is very cheap to create.
This is in preparation for the Zygote work.

[jprendes]

Wasmtime benchmark results are:
end-to-end/wasmtime/wasi-demo-app:latest: 131_303_173 ± 2_909_033 (last scheduled was 141_392_294 ± 3_279_112)
end-to-end/wasmtime/wasi-demo-oci:latest: 80_400_136 ± 1_424_339 (last scheduled was 97_305_557 ± 3_055_651)
aead_chacha20poly13052/Wasmtime: 121_648_447 ± 1_539_026 (last scheduled was 128_098_875 ± 1_606_609)

so it seems that performance is not degraded

[Mossaka]

question: why do we want to make the wasmtime engine a marker struct? You've mentioned this work is a preparation for the Zygote work. Could you please elaborate?

[jprendes]

When we introduce the zygote, we won't be able to clone the WasmtimeEngine from the shim to the child process, as the code will be executed in a different process.
This means that we need to create a new WasmtimeEngine rather than cloning.
However, we currently need a WasmtimeEngine in the shim to handle precompilation.
Making it a marker struct (i.e., a struct with no fields) means that it's not only cheap to clone, but also cheap to create.
This also allows us to use slightly different wasmtime::Engines for precompilation and for execution (e.g., we don't need pool allocator for precompilation).