changed binary date / datetime to string

[arnehormann]

Benchmark at https://gist.github.com/arnehormann/7930795 shows:
1 less allocation, speedup by factor 4+ for non-zero values

[julienschmidt]

You don't need to cache the length. The great thing about how slices and strings in Go are designed, is that getting the length is just reading an int64 - no indirection, no function calls: MOVQ src+8(FP),BX

[arnehormann]

I know. It reads smoother to me. But that's a bad habit, I'll change it.

[arnehormann]

PTAL

[julienschmidt]

LGTM, but it would be nice if we could optimize away the convT2E allocs someday.

[arnehormann]

if you have an idea how to do it, I'm game :)