Changes since 4.0.0-beta.1

Fix a couple of doc comments and an error message to say failed instead of Errored.

Summary of changes since 3.0.0

Raise minimum supported Rust version (MSRV) to 1.45.
Upgrade reqwest to 0.11 and rename feature flag to reqwest. This upgrades tokio to 1.0 and removes support for both the reqwest-010 and reqwest-09 feature flags.
Drop support for futures 0.1 and remove the futures-01 and futures-03 feature flags; only async/await and futures 0.3 are now supported (without requiring any feature flags).
Eliminate Async* traits and move the request_async methods to the underlying *Request structs
Migrate public API from http 0.1 to 0.2.
Return error types that implement std::error::Error instead of failure::Fail.
Expose a serde_path_to_error::Error<serde_json::Error>> in the RequestTokenError::Parse variant instead of a serde_json::Error to make JSON deserialization errors easier to diagnose.
Add #[non_exhaustive] attribute to AuthType to support non-breaking additions in the future.

Add support for OAuth 2.0 Device Authorization Grant.
Add support for OAuth 2.0 Token Revocation (RFC 7009) (#122).
Add support for OAuth 2.0 Token Introspection.
Add support for the plain (plaintext) PKCE verifier when the (non-default) pkce-plain feature flag is enabled. Use of this feature is discouraged for security reasons.
Support wasm32 targets (#120).
Add ureq HTTP client (#119).

Have reqwest client use rustls-tls by default instead of native TLS. This behavior can be overridden using the native-tls feature flag.
RUSTSEC-2016-0005: replace rust-crypto with hmac in dev-dependencies