Make Decodable and Decoder infallible.

`Decoder` has two impls:
- opaque: this impl is already partly infallible, i.e. in some places it
  currently panics on failure (e.g. if the input is too short, or on a
  bad `Result` discriminant), and in some places it returns an error
  (e.g. on a bad `Option` discriminant). The number of places where
  either happens is surprisingly small, just because the binary
  representation has very little redundancy and a lot of input reading
  can occur even on malformed data.
- json: this impl is fully fallible, but it's only used (a) for the
  `.rlink` file production, and there's a `FIXME` comment suggesting it
  should change to a binary format, and (b) in a few tests in
  non-fundamental ways. Indeed #85993 is open to remove it entirely.

And the top-level places in the compiler that call into decoding just
abort on error anyway. So the fallibility is providing little value, and
getting rid of it leads to some non-trivial performance improvements.

Much of this commit is pretty boring and mechanical. Some notes about
a few interesting parts:
- The commit removes `Decoder::{Error,error}`.
- `InternIteratorElement::intern_with`: the impl for `T` now has the same
  optimization for small counts that the impl for `Result<T, E>` has,
  because it's now much hotter.
- Decodable impls for SmallVec, LinkedList, VecDeque now all use
  `collect`, which is nice; the one for `Vec` uses unsafe code, because
  that gave better perf on some benchmarks.

compiler/rustc_incremental/src/persist/load.rs

@@ -158,14 +158,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
             // Decode the list of work_products
             let mut work_product_decoder = Decoder::new(&work_products_data[..], start_pos);
             let work_products: Vec<SerializedWorkProduct> =
-                Decodable::decode(&mut work_product_decoder).unwrap_or_else(|e| {
-                    let msg = format!(
-                        "Error decoding `work-products` from incremental \
-                                    compilation session directory: {}",
-                        e
-                    );
-                    sess.fatal(&msg)
-                });
+                Decodable::decode(&mut work_product_decoder);
 
             for swp in work_products {
                 let mut all_files_exist = true;
@@ -203,8 +196,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
             LoadResult::Error { message } => LoadResult::Error { message },
             LoadResult::Ok { data: (bytes, start_pos) } => {
                 let mut decoder = Decoder::new(&bytes, start_pos);
-                let prev_commandline_args_hash = u64::decode(&mut decoder)
-                    .expect("Error reading commandline arg hash from cached dep-graph");
+                let prev_commandline_args_hash = u64::decode(&mut decoder);
 
                 if prev_commandline_args_hash != expected_hash {
                     if report_incremental_info {
@@ -220,8 +212,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
                     return LoadResult::DataOutOfDate;
                 }
 
-                let dep_graph = SerializedDepGraph::decode(&mut decoder)
-                    .expect("Error reading cached dep-graph");
+                let dep_graph = SerializedDepGraph::decode(&mut decoder);
 
                 LoadResult::Ok { data: (dep_graph, prev_work_products) }
             }