Upgrade to Rust 2021

What's Changed

• Better handling of cargo-download not beeing installed by @jounathaen in #129
• enable nigthly feature "bench_black_box" by @stlankes in #130
• Rename rust-toolchain to rust-toolchain.toml by @mkroening in #131
• Disable mutable-noalias by @mkroening in #135
• Add netbench runner by @mkroening in #124
• Resolve clippy warnings by @mkroening in #138
• Rename .cargo/config to config.toml by @mkroening in #137
• Don't override kernel panic/oom handlers in userspace by @mkroening in #139
• build.rs: Prefer Command::status over output by @mkroening in #141
• Add, enforce Clippy CI by @mkroening in #140
• fix some typos by @striezel in #145
• correct repository URL for hermit-abi by @striezel in #144
• build.rs improvements by @mkroening in #143
• Rework CI by @mkroening in #142
• Fix RUSTFLAGS separator by @mkroening in #148
• revise network integration and add support of futures by @stlankes in #117
• Make NIC not mut anymore by @mkroening in #149
• collection of minor improvements for aarch64 by @stlankes in #150
• Upgrade nightly version of hermit to nightly-2021-08-31 by @mkroening in #152
• QEMU: Enable xsaveopt cpu feature by @mkroening in #155
• Migrate netbench to Rust 2018 by @mkroening in #154
• build.rs: Migrate to CARGO_ENCODED_RUSTFLAGS by @mkroening in #159
• Remove obsolete rust-toolchain files in workspace members by @mkroening in #153
• README: fix uhyve installation instructions by @Harry-R in #164
• Remove license headers by @mkroening in #165
• Upgrade to newer nightly channel by @mkroening in #171
• README: Fix Slack link by @mkroening in #173
• Update README.md by @terrarier2111 in #179
• Upgrade to Rust 2021 by @stlankes in #176
• Cargo.toml: Streamline profile configuration by @mkroening in #180
• Decouple libhermit-rs toolchain by @mkroening in #184
• Upgrade to recent nightly channel by @mkroening in #169
• Downgrade toolchain version to nightly-2021-11-18 by @mkroening in #185
• Demo: Add overaligned thread-local storage by @mkroening in #182
• build.rs: Rename aarch64 compiler_builtins symbols by @mkroening in #186

New Contributors

• @mkroening made their first contribution in #131
• @striezel made their first contribution in #145
• @Harry-R made their first contribution in #164
• @terrarier2111 made their first contribution in #179

Full Changelog: hermit-sys-0.1.21...hermit-abi-0.1.20