mapper: moved mapper to separate file, added snake case mapper

Performance is comparable to sting.ToLower

BenchmarkSnakeCase-4              200000              5922 ns/op            1216 B/op         97 allocs/op
BenchmarkToLower-4                300000              5418 ns/op             704 B/op         74 allocs/op

mapper_test.go

@@ -0,0 +1,84 @@
+package gocqlx
+
+import (
+	"strings"
+	"testing"
+)
+
+var snakeTests = []struct {
+	name     string
+	expected string
+}{
+	{"a", "a"},
+	{"snake", "snake"},
+	{"A", "a"},
+	{"ID", "id"},
+	{"MOTD", "motd"},
+	{"Snake", "snake"},
+	{"SnakeTest", "snake_test"},
+	{"APIResponse", "api_response"},
+	{"SnakeID", "snake_id"},
+	{"Snake_Id", "snake_id"},
+	{"Snake_ID", "snake_id"},
+	{"SnakeIDGoogle", "snake_id_google"},
+	{"LinuxMOTD", "linux_motd"},
+	{"OMGWTFBBQ", "omgwtfbbq"},
+	{"omg_wtf_bbq", "omg_wtf_bbq"},
+	{"woof_woof", "woof_woof"},
+	{"_woof_woof", "_woof_woof"},
+	{"woof_woof_", "woof_woof_"},
+	{"WOOF", "woof"},
+	{"Woof", "woof"},
+	{"woof", "woof"},
+	{"woof0_woof1", "woof0_woof1"},
+	{"_woof0_woof1_2", "_woof0_woof1_2"},
+	{"woof0_WOOF1_2", "woof0_woof1_2"},
+	{"WOOF0", "woof0"},
+	{"Woof1", "woof1"},
+	{"woof2", "woof2"},
+	{"woofWoof", "woof_woof"},
+	{"woofWOOF", "woof_woof"},
+	{"woof_WOOF", "woof_woof"},
+	{"Woof_WOOF", "woof_woof"},
+	{"WOOFWoofWoofWOOFWoofWoof", "woof_woof_woof_woof_woof_woof"},
+	{"WOOF_Woof_woof_WOOF_Woof_woof", "woof_woof_woof_woof_woof_woof"},
+	{"Woof_W", "woof_w"},
+	{"Woof_w", "woof_w"},
+	{"WoofW", "woof_w"},
+	{"Woof_W_", "woof_w_"},
+	{"Woof_w_", "woof_w_"},
+	{"WoofW_", "woof_w_"},
+	{"WOOF_", "woof_"},
+	{"W_Woof", "w_woof"},
+	{"w_Woof", "w_woof"},
+	{"WWoof", "w_woof"},
+	{"_W_Woof", "_w_woof"},
+	{"_w_Woof", "_w_woof"},
+	{"_WWoof", "_w_woof"},
+	{"_WOOF", "_woof"},
+	{"_woof", "_woof"},
+}
+
+func TestSnakeCase(t *testing.T) {
+	for _, tt := range snakeTests {
+		if actual := snakeCase(tt.name); actual != tt.expected {
+			t.Error("expected", tt.expected, "got", actual, tt)
+		}
+	}
+}
+
+func BenchmarkSnakeCase(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, test := range snakeTests {
+			snakeCase(test.name)
+		}
+	}
+}
+
+func BenchmarkToLower(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, test := range snakeTests {
+			strings.ToLower(test.name)
+		}
+	}
+}