[#392] ir: Define Glagolitsa alphabet type

Define `glagolicLetter` enumeration of the Glagolitsa alphabet letters.
Implement `configString` method that returns config-compatible string format
of the letters.

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

pkg/innerring/alphabet.go

@@ -0,0 +1,141 @@
+package innerring
+
+type glagoliticLetter int8
+
+const (
+	_ glagoliticLetter = iota - 1
+
+	az
+	buky
+	vedi
+	glagoli
+	dobro
+	yest
+	zhivete
+	dzelo
+	zemlja
+	izhe
+	iZhei
+	gerv
+	kako
+	ljudi
+	mislete
+	nash
+	on
+	pokoj
+	rtsi
+	slovo
+	tverdo
+	uk
+	fert
+	kher
+	oht
+	shta
+	tsi
+	cherv
+	sha
+	yer
+	yeri
+	yerj
+	yat
+	jo
+	yu
+	smallYus
+	smallIotatedYus
+	bigYus
+	bigIotatedYus
+	fita
+	izhitsa
+
+	lastLetterNum
+)
+
+// returns string in config-compatible format
+func (l glagoliticLetter) configString() string {
+	switch l {
+	default:
+		return "unknown"
+	case az:
+		return "az"
+	case buky:
+		return "buky"
+	case vedi:
+		return "vedi"
+	case glagoli:
+		return "glagoli"
+	case dobro:
+		return "dobro"
+	case yest:
+		return "yest"
+	case zhivete:
+		return "zhivete"
+	case dzelo:
+		return "dzelo"
+	case zemlja:
+		return "zemlja"
+	case izhe:
+		return "izhe"
+	case iZhei:
+		return "izhei"
+	case gerv:
+		return "gerv"
+	case kako:
+		return "kako"
+	case ljudi:
+		return "ljudi"
+	case mislete:
+		return "mislete"
+	case nash:
+		return "nash"
+	case on:
+		return "on"
+	case pokoj:
+		return "pokoj"
+	case rtsi:
+		return "rtsi"
+	case slovo:
+		return "slovo"
+	case tverdo:
+		return "tverdo"
+	case uk:
+		return "uk"
+	case fert:
+		return "fert"
+	case kher:
+		return "kher"
+	case oht:
+		return "oht"
+	case shta:
+		return "shta"
+	case tsi:
+		return "tsi"
+	case cherv:
+		return "cherv"
+	case sha:
+		return "sha"
+	case yer:
+		return "yer"
+	case yeri:
+		return "yeri"
+	case yerj:
+		return "yerj"
+	case yat:
+		return "yat"
+	case jo:
+		return "jo"
+	case yu:
+		return "yu"
+	case smallYus:
+		return "small.yus"
+	case smallIotatedYus:
+		return "small.iotated.yus"
+	case bigYus:
+		return "big.yus"
+	case bigIotatedYus:
+		return "big.iotated.yus"
+	case fita:
+		return "fita"
+	case izhitsa:
+		return "izhitsa"
+	}
+}