Move DataType into internal/datatype, aliased by the root, and what a proof knows of a value into internal/proven

internal/datatype/datatype.go

@@ -0,0 +1,34 @@
+// Package datatype names what a record column holds.
+package datatype
+
+import "fmt"
+
+// DataType is what a record column holds, which decides how a record writes its value.
+type DataType int
+
+// The datatypes a column declares with "datatype"; a column without one is a string.
+const (
+	String DataType = iota
+	Integer
+	Number
+	Boolean
+)
+
+// Count is the number of datatypes, for an array indexed by one.
+const Count = len(names)
+
+var (
+	names = [...]string{"string", "integer", "number", "boolean"}
+	nouns = [...]string{"text", "an integer", "a number", "a boolean"}
+)
+
+// String is the datatype as data spells it.
+func (d DataType) String() string {
+	if d < 0 || int(d) >= len(names) {
+		return fmt.Sprintf("DataType(%d)", int(d))
+	}
+	return names[d]
+}
+
+// Noun is the datatype as an error names what a value is.
+func Noun(d DataType) string { return nouns[d] }