Strip all whitespace from a string

Here is some benchmarks on a few different methods for stripping all whitespace characters from a string: (source data):

BenchmarkSpaceMap-8                     2000       1100084 ns/op      221187 B/op          2 allocs/op
BenchmarkSpaceFieldsJoin-8              1000       2235073 ns/op     2299520 B/op         20 allocs/op
BenchmarkSpaceStringsBuilder-8          2000        932298 ns/op      122880 B/op          1 allocs/op

• SpaceMap: uses strings.Map; gradually increases the amount of allocated space as more non-whitespace characters are encountered
• SpaceFieldsJoin: strings.Fields and strings.Join; generates a lot of intermediate data
• SpaceStringsBuilder uses strings.Builder; performs a single allocation, but may grossly overallocate if the source string is mainly whitespace.

package main_test

import (
    "strings"
    "unicode"
    "testing"
)

func SpaceMap(str string) string {
    return strings.Map(func(r rune) rune {
        if unicode.IsSpace(r) {
            return -1
        }
        return r
    }, str)
}

func SpaceFieldsJoin(str string) string {
    return strings.Join(strings.Fields(str), "")
}

func SpaceStringsBuilder(str string) string {
    var b strings.Builder
    b.Grow(len(str))
    for _, ch := range str {
        if !unicode.IsSpace(ch) {
            b.WriteRune(ch)
        }
    }
    return b.String()
}

func BenchmarkSpaceMap(b *testing.B) {
    for n := 0; n < b.N; n++ {
        SpaceMap(data)
    }
}

func BenchmarkSpaceFieldsJoin(b *testing.B) {
    for n := 0; n < b.N; n++ {
        SpaceFieldsJoin(data)
    }
}

func BenchmarkSpaceStringsBuilder(b *testing.B) {
    for n := 0; n < b.N; n++ {
        SpaceStringsBuilder(data)
    }
}