Fix clippy lints

Specifically `rust-version = "1.58"` in the `autogen` module that
I added in #223 is weird, since any developers working on and/or
contributing to the repository should easily be able to run the latest
stable and take advantage of newer Rust features in `autogen` code.

autogen/Cargo.toml

@@ -6,7 +6,6 @@ authors = [
 	"Lei Zhang <[email protected]>",
 ]
 edition = "2018"
-rust-version = "1.58"
 
 publish = false
 

autogen/src/dr.rs

@@ -391,7 +391,7 @@ pub fn gen_dr_operand_kinds(grammar: &[structs::OperandKind]) -> TokenStream {
                 let mut seen_discriminator = BTreeMap::new();
 
                 for e in enumerators {
-                    if seen_discriminator.get(&e.value).is_none() {
+                    if let std::collections::btree_map::Entry::Vacant(seen_entry) = seen_discriminator.entry(e.value) {
                         let name = match category {
                             structs::Category::BitEnum => {
                                 use heck::ShoutySnakeCase;
@@ -412,7 +412,7 @@ pub fn gen_dr_operand_kinds(grammar: &[structs::OperandKind]) -> TokenStream {
                             _ => panic!("Unexpected operand type"),
                         };
 
-                        seen_discriminator.insert(e.value, name.clone());
+                        seen_entry.insert(name.clone());
 
                         capability_clauses
                             .entry(&e.capabilities)