text/template: line number tracking is O(N^2)

Per the golang-dev thread, the text/template line tracking is O(N^2):

```go
// lineNumber reports which line we're on, based on the position of
// the previous item returned by nextItem. Doing it this way
// means we don't have to worry about peek double counting.
func (l *lexer) lineNumber() int {
	return 1 + strings.Count(l.input[:l.lastPos], "\n")
}
```

Tracking bug for @robpike.
