Remove duplicated function definitions

[cespare]

After Go 1.13 comes out, we can freely use "forwarding" functions and avoid duplicated definitions since these trivial mid-stack functions should be inlined now: https://github.com/golang/go/issues/8421

[cespare]

I did look into this after Go 1.13 came out and noticed that adding back the forwarding functions caused the benchmarks to get slower. I didn't look into it much more.

I took a closer look today and discovered that the situation is more nuanced (at least with Go 1.15):

• With a direct call, the forwarding function is inlined away and becomes free.
• With an indirect call, the forwarding function cannot be inlined and incurs a small cost: https://github.com/golang/go/issues/32577

The benchmarks are written to be kind of generic and use indirect calls. But I suspect anyone who cares about spending an extra 2ns hashing a small string is using direct calls. So I'm tempted to add the forwarding functions and also rewrite the benchmarks to not use indirect calls.

[cespare]

I did the benchmarking change in e0ea1e3196ac0aac60eccd7670662049875d8b7b.

Just in case someone does care, I'll make the forwarding function change as part of v3.

[greatroar]

The comparison benchmarks can do direct calls if they're written with build tags. I did that in my Bloom filter package.