Resolve unnecessary_get_then_check clippy lint

    warning: unnecessary use of `get(resolved).is_some()`
       --> src/parse.rs:172:37
        |
    172 |                     if lookup.items.get(resolved).is_some() {
        |                                     ^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `contains_key(resolved)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check
        = note: `-W clippy::unnecessary-get-then-check` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::unnecessary_get_then_check)]`

codegen/src/parse.rs

@@ -169,7 +169,7 @@ fn introspect_type(item: &syn::Type, lookup: &Lookup) -> types::Type {
                     while let Some(alias) = lookup.aliases.get(resolved) {
                         resolved = alias;
                     }
-                    if lookup.items.get(resolved).is_some() {
+                    if lookup.items.contains_key(resolved) {
                         types::Type::Syn(resolved.to_string())
                     } else {
                         unimplemented!("{}", resolved);