[#316] locode: Define UN/LOCODE table data types

Define the data types needed to work with LOCODE's in NeoFS (country code,
location code, coordinates). Implement string parsers for new types.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/locode/column/country.go

@@ -0,0 +1,33 @@
+package locodecolumn
+
+import (
+	"github.com/nspcc-dev/neofs-node/pkg/util/locode"
+)
+
+const countryCodeLen = 2
+
+// CountryCode represents ISO 3166 alpha-2 Country Code.
+type CountryCode [countryCodeLen]uint8
+
+// Symbols returns digits of the country code.
+func (cc *CountryCode) Symbols() [countryCodeLen]uint8 {
+	return *cc
+}
+
+// CountryCodeFromString parses string and returns country code.
+func CountryCodeFromString(s string) (*CountryCode, error) {
+	if len(s) != countryCodeLen {
+		return nil, locode.ErrInvalidString
+	}
+
+	for i := range s {
+		if !isUpperAlpha(s[i]) {
+			return nil, locode.ErrInvalidString
+		}
+	}
+
+	cc := CountryCode{}
+	copy(cc[:], s)
+
+	return &cc, nil
+}