build: justify hardcoded build.rs RUSTLS_CRATE_VERSION

This commit adds some commentary to the `build.rs`
`RUSTLS_CRATE_VERSION` const, justifying why we keep it in-sync
manually.

Initially I was thinking about deducing the `RUSTLS_CRATE_VERSION` in
the `build.rs` script automatically by parsing `Cargo.toml`, but after
thinking through the sensitivity of `build.rs`, and realizing the
parsing would require a new _non-development_ dependency, I abandoned
the idea.

The recently added integration test will catch anyone forgetting to sync
the const and avoids extra deps or complexity in the build script.

build.rs

@@ -3,6 +3,11 @@ use std::io::Write;
 use std::{env, fs, path::PathBuf};
 
 // Keep in sync with Cargo.toml.
+//
+// We don't populate this automatically from the Cargo.toml at build time
+// because doing so would require a heavy-weight deserialization lib dependency
+// (and it couldn't be a _dev_ dep for use in a build script) or doing brittle
+// by-hand parsing.
 const RUSTLS_CRATE_VERSION: &str = "0.23.4";
 
 fn main() {