feat: add nil backend

[haraldh]

This backend calls exec-wasmtime directly via a library call, without using any elf binary and shim.

❯ enarx run --backend nil hello_wasi_snapshot1.wasm
Hello, world!

[codecov]

Codecov Report

Merging #1794 (25c8a9f) into main (ea6d853) will increase coverage by 2.33%.
The diff coverage is 70.76%.

❗ Current head 25c8a9f differs from pull request most recent head bfe0286. Consider uploading reports for the commit bfe0286 to get more accurate results

@@            Coverage Diff             @@
##             main    #1794      +/-   ##
==========================================
+ Coverage   54.68%   57.01%   +2.33%     
==========================================
  Files         151      152       +1     
  Lines       10316    10352      +36     
==========================================
+ Hits         5641     5902     +261     
+ Misses       4675     4450     -225     

Impacted Files	Coverage Δ
src/backend/direct.rs	65.45% <65.45%> (ø)
crates/exec-wasmtime/src/lib.rs	79.62% <100.00%> (ø)
src/backend/mod.rs	54.54% <100.00%> (+4.54%)	⬆️
src/main.rs	95.00% <100.00%> (+2.14%)	⬆️
src/workldr/exec_wasmtime/mod.rs	78.57% <100.00%> (ø)
src/backend/sgx/hasher.rs	0.00% <0.00%> (ø)
src/backend/sgx/builder.rs	91.97% <0.00%> (ø)
...es/exec-wasmtime/src/loader/configured/platform.rs	84.12% <0.00%> (+6.34%)	⬆️
crates/exec-wasmtime/src/loader/mod.rs	87.80% <0.00%> (+7.31%)	⬆️
... and 6 more

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[npmccallum]

@platten Note that this makes the "static binary" problem much more acute since the dependency graphs are now overlapping.

[bstrie]

@npmccallum Can you elaborate on how this makes the static binary problem more acute?

[bstrie]

So AIUI the reason to split out exec-wasmtime into lib and bin crates is so that we can make the lib optional and avoid building wasmtime twice, once for the host and once for musl, since wasmtime takes a while to build. @npmccallum This isn't a new bug in bindeps, because it's expected behavior; bindeps allows you to have both an artifact and a normal dependency via the lib = true key, but there's no facility to make lib = true optional, and it's unclear how you'd even expose that. So this is bindeps working as intended, and is not relevant to the known optional = true bug ( rust-lang/cargo#10526 ).

Instead, the ultimate fix for this would be multidep ( rust-lang/cargo#10061 ), which would allow us to have a single exec-wasmtime crate and rely on it twice from within Cargo.toml, once as an optional direct dependency and once as a bindep. Please leave a FIXME somewhere mentioning that we can undo this split once multidep arrives.

[haraldh]

Shouldn't we open an issue which tracks the external multidep and a second one depending on this to fix our monorepo?

[bstrie]

@haraldh #1798 and #1799. Can you add the first as a FIXME comment?

[haraldh]

@haraldh #1798 and #1799. Can you add the first as a FIXME comment?

@bstrie done