Split arg-to-param matching into a separate function

Could be faster with templates but seems roughly even for now.

src/codegen/ast_interpreter.cpp

@@ -752,7 +752,7 @@ Value ASTInterpreter::visit_stmt(AST_stmt* node) {
 #endif
 
     if (0) {
-        printf("%20s % 2d ", source_info->getName().c_str(), current_block->idx);
+        printf("%20s % 2d ", source_info->getName().data(), current_block->idx);
         print_ast(node);
         printf("\n");
     }

src/codegen/irgen/hooks.cpp

@@ -87,7 +87,7 @@ InternedStringPool& SourceInfo::getInternedStrings() {
     return scoping->getInternedStrings();
 }
 
-const std::string SourceInfo::getName() {
+llvm::StringRef SourceInfo::getName() {
     assert(ast);
     switch (ast->type) {
         case AST_TYPE::ClassDef:

src/core/cfg.cpp

@@ -2595,7 +2595,7 @@ CFG* computeCFG(SourceInfo* source, std::vector<AST_stmt*> body) {
         if (b->predecessors.size() == 0) {
             if (b != rtn->getStartingBlock()) {
                 rtn->print();
-                printf("%s\n", source->getName().c_str());
+                printf("%s\n", source->getName().data());
             }
             ASSERT(b == rtn->getStartingBlock(), "%d", b->idx);
         }

src/core/types.h

@@ -300,7 +300,7 @@ public:
     // body and we have to create one.  Ideally, we'd be able to avoid the space duplication for non-lambdas.
     const std::vector<AST_stmt*> body;
 
-    const std::string getName();
+    llvm::StringRef getName();
     InternedString mangleName(InternedString id);
 
     Box* getDocString();