Add home_dir for WASI

[HKalbasi]

This PR adds home_dir implementation for Web Assembly System Interface (WASI) target, that uses HOME environment variable. Since WASI lacks concept of user, I think this is the way to go.

Lacking this function makes porting binaries to WASI unnecessarily harder.

[LucioFranco]

@HKalbasi hi! sorry for the delay but do you have a doc or reference for what wasi sets?

[HKalbasi]

I think, ..., it doesn't really matter? WASI doesn't have concept of users, and its filesystem is isolated and sandboxed, by using $HOME codes will compile and people can change it to whatever they want at runtime (probably /). From what I have seen before, WASI is more close to unix than windows so I think $HOME is good choice.

But to make sure, I opened WebAssembly/wasi-filesystem#59

[turbolent]

@brson Could this PR please get merged? Some applications, like https://github.com/helix-editor/helix, depend on this crate, and compilation of the home crate for WASI fails at the moment.

Assuming the HOME environment variable is set seems reasonable – WASI is close to POSIX.

[HKalbasi]

@turbolent It is not clear which solution is the correct one for WASI. You might want to say your opinion on WebAssembly/wasi-filesystem#59

[turbolent]

Well, having a possible implementation which is close to what other platforms do is still better than none. Given there is no "correct" one. Currently, the missing implementation means applications using it do not build at all.

[HKalbasi]

The suggested implementation in that issue is unconditionally returning None, which doesn't work for my use case. If it works for helix, I think it can also make home option dependency and cfg it out.

[LucioFranco]

Btw all maintenance of this project has moved into cargo's repository.