*: Drop generic wasm32-unknown-unknown support

[mxinden]

With rand v0.8.0 platform support changed [1] due to its upgrade to
getrandom v0.2. With getrandom v0.2 wasm32-unknown-unknown is no
longer supported out of the box:

This crate fully supports the wasm32-wasi and wasm32-unknown-emscripten
targets. However, the wasm32-unknown-unknown target is not automatically
supported since, from the target name alone, we cannot deduce which JavaScript
interface is in use (or if JavaScript is available at all).

Instead, if the "js" Cargo feature is enabled, this crate will assume that you
are building for an environment containing JavaScript, and will call the
appropriate methods. Both web browser (main window and Web Workers) and
Node.js environments are supported, invoking the methods described above using
the wasm-bindgen toolchain.

This feature has no effect on targets other than wasm32-unknown-unknown.

This commit drops support for wasm32-unknown-unknown in favor of the two more
specific targets wasm32-wasi and wasm32-unknown-emscripten.

Note on resolver = "2": The new resolver is required to prevent features
being mixed, more specifically to prevent libp2p-noise to build with the
ring-resolver feature. See [3] for details.

---

[1] https://github.com/rust-random/rand/blob/master/CHANGELOG.md#platform-support
[2] https://docs.rs/getrandom/0.2.2/getrandom/#webassembly-support
[3] https://doc.rust-lang.org/nightly/cargo/reference/features.html#feature-resolver-version-2

[tomaka]

After reading into the details, getrandom v0.2 doesn't actually change anything. It renames the wasm-bindgen and stdweb features to js, but doesn't fundamentally stops supporting wasm32-unknown-unknown.

A user of rust-libp2p can add an explicit dependency to getrandom in their Cargo.toml in order to enable the feature. This is exactly the same as before.

I believe it makes sense to no longer try to maintain this weird unknown variant that doesn't really make sense, so +1 for this PR. However, this PR doesn't make rust-libp2p stop working on that platform.

[mxinden]

Thanks @tomaka for the review.

A user of rust-libp2p can add an explicit dependency to getrandom in their Cargo.toml in order to enable the feature. This is exactly the same as before.

I believe it makes sense to no longer try to maintain this weird unknown variant that doesn't really make sense, so +1 for this PR. However, this PR doesn't make rust-libp2p stop working on that platform.

👍