[#316] locode: Define the API of location database

Define structure of keys and records of the location database. Define the
interfaces of all components necessary for the formation of the database.
Implement the function of filling the database.

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

pkg/util/locode/db/continent.go

@@ -0,0 +1,81 @@
+package locodedb
+
+// Continent is an enumeration of Earth's continent.
+type Continent uint8
+
+const (
+	// ContinentUnknown is an undefined Continent value.
+	ContinentUnknown = iota
+
+	// ContinentEurope corresponds to Europe.
+	ContinentEurope
+
+	// ContinentAfrica corresponds to Africa.
+	ContinentAfrica
+
+	// ContinentNorthAmerica corresponds to North America.
+	ContinentNorthAmerica
+
+	// ContinentSouthAmerica corresponds to South America.
+	ContinentSouthAmerica
+
+	// ContinentAsia corresponds to Asia.
+	ContinentAsia
+
+	// ContinentAntarctica corresponds to Antarctica.
+	ContinentAntarctica
+
+	// ContinentOceania corresponds to Oceania.
+	ContinentOceania
+)
+
+// Is checks if c is the same continent as c2.
+func (c *Continent) Is(c2 Continent) bool {
+	return *c == c2
+}
+
+func (c Continent) String() string {
+	switch c {
+	case ContinentUnknown:
+		fallthrough
+	default:
+		return "Unknown"
+	case ContinentEurope:
+		return "Europe"
+	case ContinentAfrica:
+		return "Africa"
+	case ContinentNorthAmerica:
+		return "North America"
+	case ContinentSouthAmerica:
+		return "South America"
+	case ContinentAsia:
+		return "Asia"
+	case ContinentAntarctica:
+		return "Antarctica"
+	case ContinentOceania:
+		return "Oceania"
+	}
+}
+
+// ContinentFromString returns Continent value
+// corresponding to passed string representation.
+func ContinentFromString(str string) Continent {
+	switch str {
+	default:
+		return ContinentUnknown
+	case "Europe":
+		return ContinentEurope
+	case "Africa":
+		return ContinentAfrica
+	case "North America":
+		return ContinentNorthAmerica
+	case "South America":
+		return ContinentSouthAmerica
+	case "Asia":
+		return ContinentAsia
+	case "Antarctica":
+		return ContinentAntarctica
+	case "Oceania":
+		return ContinentOceania
+	}
+}