strftime: faster StrftimePure

With the new benchmarks, here's the old implementation (on my '13 MBP
Retina, 2.3GHz i7 4-core):
BenchmarkStrftimePure     100000  14621 ns/op  4808 B/op   22 allocs/op

And the new:
BenchmarkStrftimePure     500000  3230 ns/op    304 B/op    4 allocs/op

On the same machine, within VMWare, the Linux benchmarks:

BenchmarkStrftime         500000  3815 ns/op    393 B/op    7 allocs/op
BenchmarkStrftimePure     500000  3141 ns/op    304 B/op    4 allocs/op