Default all async support to std::future

This commit defaults all crates in-tree to use `std::future` by default
and none of them support the crates.io `futures` 0.1 crate any more.
This is a breaking change for `wasm-bindgen-futures` and
`wasm-bindgen-test` so they've both received a major version bump to
reflect the new defaults. Historical versions of these crates should
continue to work if necessary, but they won't receive any more
maintenance after this is merged.

The movement here liberally uses `async`/`await` to remove the need for
using any combinators on the `Future` trait. As a result many of the
crates now rely on a much more recent version of the compiler,
especially to run tests.

The `wasm-bindgen-futures` crate was updated to remove all of its
futures-related dependencies and purely use `std::future`, hopefully
improving its compatibility by not having any version compat
considerations over time. The implementations of the executors here are
relatively simple and only delve slightly into the `RawWaker` business
since there are no other stable APIs in `std::task` for wrapping these.

This commit also adds support for:

    #[wasm_bindgen_test]
    async fn foo() {
        // ...
    }

where previously you needed to pass `(async)` now that's inferred
because it's an `async fn`.

Closes rustwasm#1558
Closes rustwasm#1695

Cargo.toml

@@ -43,7 +43,7 @@ cfg-if = "0.1.9"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 js-sys = { path = 'crates/js-sys', version = '0.3.27' }
-wasm-bindgen-test = { path = 'crates/test', version = '=0.2.50' }
+wasm-bindgen-test = { path = 'crates/test', version = '=0.3.0' }
 serde_derive = "1.0"
 wasm-bindgen-test-crate-a = { path = 'tests/crates/a', version = '0.1' }
 wasm-bindgen-test-crate-b = { path = 'tests/crates/b', version = '0.1' }

crates/futures/Cargo.toml

@@ -7,17 +7,13 @@ license = "MIT/Apache-2.0"
 name = "wasm-bindgen-futures"
 repository = "https://github.com/rustwasm/wasm-bindgen/tree/master/crates/futures"
 readme = "./README.md"
-version = "0.3.27"
+version = "0.4.0"
 edition = "2018"
 
 [dependencies]
 cfg-if = "0.1.9"
-futures = "0.1.20"
 js-sys = { path = "../js-sys", version = '0.3.27' }
 wasm-bindgen = { path = "../..", version = '0.2.50' }
-futures-util-preview = { version = "0.3.0-alpha.18", optional = true }
-futures-channel-preview = { version = "0.3.0-alpha.18", optional = true }
-lazy_static = { version = "1.3.0", optional = true }
 
 [target.'cfg(target_feature = "atomics")'.dependencies.web-sys]
 path = "../web-sys"
@@ -28,7 +24,4 @@ features = [
 ]
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
-wasm-bindgen-test = { path = '../test', version = '0.2.50' }
-
-[features]
-futures_0_3 = ["futures-util-preview", "futures-channel-preview", "lazy_static"]
+wasm-bindgen-test = { path = '../test', version = '0.3.0' }