Assert that sysroot ABI version matches exactly

Otherwise, fall back to the multi ABI scheme, except in testing, where
it becomes a hard error.

This should make it possible to use a rustup-provided rust-analyzer with
proc macro dylibs compiled by older rustcs, and it'll also catch changes
to the format of `rustc --version` or the `.rustc` section that would
make them impossible to compare for equality.

crates/proc-macro-api/src/lib.rs

@@ -26,7 +26,7 @@ use crate::{
     process::ProcMacroProcessSrv,
 };
 
-pub use version::{read_dylib_info, RustCInfo};
+pub use version::{read_dylib_info, read_version, RustCInfo};
 
 #[derive(Copy, Clone, Eq, PartialEq, Debug, Serialize, Deserialize)]
 pub enum ProcMacroKind {