Coalesce inline and normal alias headers

Unifies `As` annotations and `Alias` defs to use a common struct for
defining alias header information, i.e. the name and type variables of
the alias.

compiler/load/src/docs.rs

@@ -6,8 +6,8 @@ use crate::file::LoadedModule;
 use roc_can::scope::Scope;
 use roc_module::ident::ModuleName;
 use roc_module::symbol::IdentIds;
-use roc_parse::ast;
 use roc_parse::ast::CommentOrNewline;
+use roc_parse::ast::{self, AliasHeader};
 use roc_parse::ast::{AssignedField, Def};
 use roc_region::all::Loc;
 
@@ -205,7 +205,10 @@ fn generate_entry_doc<'a>(
             _ => (acc, None),
         },
 
-        Def::Alias { name, vars, ann } => {
+        Def::Alias {
+            header: AliasHeader { name, vars },
+            ann,
+        } => {
             let mut type_vars = Vec::new();
 
             for var in vars.iter() {