Add benchmarks for GoDecimal vs inf.Dec

### Code Review

Benchmarks for `goDecimal` vs `inf.Dec`.

```
$ go test -bench . -benchmem
PASS
BenchmarkGoDecimal_FromString-8	 1000000	      1262 ns/op	     570 B/op	       6 allocs/op
BenchmarkInfDec_FromString-8	 1000000	      1011 ns/op	     198 B/op	       5 allocs/op
ok  	github.com/matthinrichsen-wf/goDecimal	2.303s
```

Not bad considering `inf.Dec` doesn't support exponential notation and this is still a rough iteration. The memory footprint is probably unavoidable at this stage because goDecimal uses a slice instead of a `big.Int`.

@matthinrichsen-wf