Start to update the wasi crate in wasi tests

[alexcrichton]

This commit starts an update where all the wasi-tests/src/bin/*.rs programs are updated to the newest snapshot of the wasi API, rather tan using the old snapshot 0.

Not all tests are updated, and there's a few changes here:

• The test harness instantiates both snapshots of the wasi API now
• Both 0.7.0 and 0.9.0 are available to tests. The 0.7.0 version is now known as wasi_old and the 0.9.0 version is now known as just normal wasi.
• A few tests have been updated to use wasi rather than wasi_old
• The open_scratch_directory function had to get a duplicate which uses the most recent snapshot's file descriptors.

The purpose of this PR is to start us down the road of updating these tests, allowing the update to happen in parallel for each test. Also to get some early feedback!

[sunfishcode]

I didn't know one could depend on two versions of the same crate at once, cool!

[kubkon]

All in all, this looks great to me, awesome job, thanks @alexcrichton! Just to strengthen my understanding though, did I get it right that we instantiate both snapshots as two Wasm modules, and then a test picks the correct one at runtime?

[alexcrichton]

Just to strengthen my understanding though, did I get it right that we instantiate both snapshots as two Wasm modules, and then a test picks the correct one at runtime?

Indeed!

Basically both snapshots are available under their respective modules, and depending on the name imported the test gets the respective functionality.

[alexcrichton]

Updated!

[kubkon]

Just to strengthen my understanding though, did I get it right that we instantiate both snapshots as two Wasm modules, and then a test picks the correct one at runtime?

Indeed!

Basically both snapshots are available under their respective modules, and depending on the name imported the test gets the respective functionality.

Hmm, after careful re-read of your comments in the issue referenced, I'm thinking it would be worth finishing what you've started here and rewriting all of the remaining tests to indeed use wasi crate. I think I should be able to do this pretty easily (here's me hoping I don't underestimate the magnitude of the task!). What do you think?

[kubkon]

Updated!

LGTM! I'm happy to merge it as-is!

[kubkon]

I've rebased to current master and fast-forwarded to latest Wasmtime API changes, and will merge as-is.