Removed more string conversions and streamlined bottleneck
printing interface by removing unnecessary tests where possible.
About 6% faster AST printing.

Before:
- printer.BenchmarkPrint		50	32056640 ns/op

After:
- printer.BenchmarkPrint		50	30138440 ns/op (-6%)

R=r
CC=golang-dev
https://golang.org/cl/5431047