Split defer cost out of BenchmarkSuccess

[dsnet]

This allows us to measure the cost of E and Handle seperately.

[dsnet]

For the record, this prints:

BenchmarkDefer-24       100000000           10.66 ns/op        0 B/op          0 allocs/op
BenchmarkSuccess-24     1000000000           0.5143 ns/op          0 B/op          0 allocs/op
BenchmarkFailure-24      2998248           399.1 ns/op        24 B/op          1 allocs/op

That's good news since it means that the E functions are really cheap.

Defers are pretty expensive, and most of the cost seems to be in the defer implementation itself. I didn't look too much into it.