Change the MSRV policy and bump to 1.79.0 (#480)

The MSRV is being bumped so that `domain` can access newer language features.  In particular:

- `<[u8]>::utf8_chunks()` (1.79) is very useful for processing byte strings that will _mostly_ contain valid UTF-8 text.
- `core::net` (1.77) allows us to remove our polyfills of `Ipv4Addr` etc., used when the `std` feature is disabled.
- `async fn` and return-position `impl Trait` in traits (1.75) allow us to define traits using `async fn`, avoiding the overhead of `Box<dyn Future>` for un-nameable future types.
- `Option::is_some_and()` (1.70) simplifies a very common pattern when inspecting optional values.

The MSRV bump policy is also being updated; it sets a maximum MSRV (relative to the latest stable Rust version) and documents the conditions under which a bump will occur.

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        rust: [1.76.0, stable, beta, nightly]
+        rust: [1.79.0, stable, beta, nightly]
     env:
         RUSTFLAGS: "-D warnings"
         # We use 'vcpkg' to install OpenSSL on Windows.
@@ -57,7 +57,7 @@ jobs:
     - name: Install Rust
       uses: hecrj/setup-rust-action@v2
       with:
-        rust-version: "1.68.2"
+        rust-version: "1.79.0"
     - name: Install OpenSSL
       run: sudo apt-get install -y libssl-dev
     - name: Install nightly Rust
@@ -81,7 +81,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [1.78.0, stable, beta, nightly]
+        rust: [1.79.0, stable, beta, nightly]
     steps:
     - name: Checkout repository
       uses: actions/checkout@v1