schema: split 'column', 'index' and 'schema' into separate files from 'table'

pkg/schema/index.go

@@ -0,0 +1,10 @@
+package schema
+
+type Index struct {
+	Name      string `db:"index_name"`
+	TableName string `db:"table_name"`
+	Columns   []string
+	IsUnique  bool `db:"is_unique"`
+	// TODO: `where ...` for partial indexes
+	// TODO: identify columns that are expressions
+}