Initial support for generic parameters that are introduced in a function parameter list (#1247)

In order for this to work, we need to always use the argument deduction code path for function calls, instead of only using it when there is a `[...]` list, which means that argument deduction now needs to support implicit conversion.

Co-authored-by: Jon Meow <jperkins@google.com>

explorer/ast/declaration.cpp

@@ -198,9 +198,9 @@ auto FunctionDeclaration::Create(
                << "illegal AST node in implicit parameter list";
     }
   }
-  return arena->New<FunctionDeclaration>(source_loc, name, resolved_params,
-                                         me_pattern, param_pattern, return_term,
-                                         body);
+  return arena->New<FunctionDeclaration>(source_loc, name,
+                                         std::move(resolved_params), me_pattern,
+                                         param_pattern, return_term, body);
 }
 
 void FunctionDeclaration::PrintDepth(int depth, llvm::raw_ostream& out) const {