Make date a structured type

This introduces a convening way to access the year, month, and day
parts of date objects, making the year(), month(), and day()
functions, and the homonym columns of the postings and entries tables
redundant. These will be deprecated and eventually removed.

beanquery/query_env.py

@@ -590,6 +590,27 @@ def possign(context, x, account):
     return x if sign >= 0  else -x
 
 
+# ``date`` type
+
+class Date(types.Structure):
+    name = 'date'
+    columns = ColumnsRegistry()
+
+    @columns.register(int)
+    def year(x):
+        return x.year
+
+    @columns.register(int)
+    def month(x):
+        return x.month
+
+    @columns.register(int)
+    def day(x):
+        return x.day
+
+types.ALIASES[datetime.date] = Date
+
+
 @function([str], datetime.date)
 @function([str, str], datetime.date)
 def parse_date(string, frmt=None):