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.go

@@ -0,0 +1,40 @@
+package gocqlx
+
+import (
+	"fmt"
+	"unicode"
+
+	"github.com/jmoiron/sqlx/reflectx"
+)
+
+// DefaultMapper uses `db` tag and automatically converts struct field names to
+// snake case. It can be set to whatever you want, but it is encouraged to be
+// set before gocqlx is used as name-to-field mappings are cached after first
+// use on a type.
+var DefaultMapper = reflectx.NewMapperFunc("db", snakeCase)
+
+// snakeCase converts camel case to snake case.
+func snakeCase(s string) string {
+	buf := []byte(s)
+	out := make([]byte, 0, len(buf)+3)
+
+	l := len(buf)
+	for i := 0; i < l; i++ {
+		if !(allowedBindRune(buf[i]) || buf[i] == '_') {
+			panic(fmt.Sprint("not allowed name ", s))
+		}
+
+		b := rune(buf[i])
+
+		if unicode.IsUpper(b) {
+			if i > 0 && buf[i-1] != '_' && (unicode.IsLower(rune(buf[i-1])) || (i+1 < l && unicode.IsLower(rune(buf[i+1])))) {
+				out = append(out, '_')
+			}
+			b = unicode.ToLower(b)
+		}
+
+		out = append(out, byte(b))
+	}
+
+	return string(out)
+}