Explorer: Add methods to trace for line prefixes (#3076)

Defines methods into `TraceStream` for adding line prefixes. Instead of
directly using string literals.

Example usage,
```
trace_stream_->Start() << "declaring ... " << ... ;
```
will result in,
```
->> declaring ... 
```

explorer/interpreter/impl_scope.cpp

@@ -416,7 +416,7 @@ auto ImplScope::TryResolveInterfaceHere(
 // TODO: Add indentation when printing the parents.
 void ImplScope::Print(llvm::raw_ostream& out) const {
   llvm::ListSeparator sep(",\n    ");
-  out << " +  "
+  out << "    "
       << "[";
   for (const ImplFact& impl : impl_facts_) {
     out << sep << "`" << *(impl.type) << "` as `" << *(impl.interface) << "`";