Minimize allocations in strutil

[sethvargo]

I noticed some pretty quick-and-easy wins for performance in the strutil package.

1. Use struct{} instead of bool for map membership. The empty struct does not allocate and consumes no memory, so it's far more efficient than a boolean:

   package main
   
   import (
       "fmt"
       "unsafe"
   )
   
   func main() {
       x := [100]struct{}{}
       y := [100]bool{}
   
       fmt.Printf("%v\n", unsafe.Sizeof(x)) // 0
       fmt.Printf("%v\n", unsafe.Sizeof(y)) // 100
   }

2. Pre-allocate slices and maps when we know the maximum possible size.

3. Remove an unnecessary test dependency and rename the tests to match Go's expected test format name.

Benchmarks

Before:

BenchmarkRemoveDuplicates-16                   2     756790682 ns/op    106821536 B/op     38192 allocs/op
BenchmarkRemoveDuplicatesStable-16             6     183990038 ns/op    58343824 B/op          4 allocs/op
BenchmarkEquivalentSlices-16                   1    1666107197 ns/op    358676792 B/op     76680 allocs/op

After:

BenchmarkRemoveDuplicates-16                   3     477817660 ns/op    56117237 B/op          7 allocs/op
BenchmarkRemoveDuplicatesStable-16             6     200917214 ns/op    56115224 B/op          3 allocs/op
BenchmarkEquivalentSlices-16                   1    1009316412 ns/op    112230496 B/op         8 allocs/op

[sethvargo]

Pokey @jefferai @calvn

[jefferai]

I'm on vacation :-)

[jefferai]

Thanks!

[sethvargo]

Thank you 😄