Fix race condition surrounding global orgWriter

Since writers are normally only used synchronously (i.e. to write one document
at a time), we don't guard modifications to their internal
state (e.g. temporarily replacing the string.Builder in WriteNodesAsString)
against race conditions.

The package global `orgWriter` and corresponding use cases of it (`org.String`,
`$node.String`) break that pattern - the writer is potentially used from
multiple go routines at the same time. This results in race conditions that
manifest as error messages like e.g.

    could not write output: runtime error: invalid memory address or nil pointer dereference. Using unrendered content.

Additionally, since we catch panics in `Document.Write`, the corresponding
stack trace is lost and dependents of go-org never know what hit them.

As using a writer across simultaneously across go routines is not a standard
pattern, we'll sync the use of the global `orgWriter` instead of trying to make
the actual writer threadsafe; less code noise for the common use case.

org/headline.go

@@ -126,4 +126,4 @@ func (parent *Section) add(current *Section) {
 	}
 }
 
-func (n Headline) String() string { return orgWriter.WriteNodesAsString(n) }
+func (n Headline) String() string { return String(n) }