Revert tinygo reflect.SliceHeader specialization

[ncruces]

In light of tinygo-org/tinygo#4156, we need to revert #2161.

Also use unsafe.Add where feasible, and remove TODOs to use unsafe.Slice where doing so would raise a go vet warning about misuse of unsafe.Pointer.

[ncruces]

Drafting while waiting for the TinyGo release, to update failed integration tests.

[deadprogram]

We are in process of releasing v0.32 so this PR should be prepared, please.

[ncruces]

Thanks for the heads up.

I think this is mostly ready, I just need to update CI to download the v0.32 binaries, and run tests.

[ncruces]

Linux binaries still missing from here, will try again.

[ncruces]

@deadprogram it seems TinyGo v0.32 lost the ability to build --target=wasi binaries with Go 1.20?

[ncruces]

Idk how we feel about skipping building tests with Go 1.20 + TinyGo v0.32.0, as that seems to be an unsupported combination.

[mathetake]

yeah looks like a bug in TinyGo that -target=wasi is now trying to use GOOS=wasip1, which is not existent in Go 1.20, for source resolution vs previously it didn't use wasip1 target internally (I remember it used arm target virtually). I think anyway this is harmless so I will go ahead and merge. cc @deadprogram my guess is that the latest TinyGo breaks wasi builds for all Go versions <= 1.20.