data: Define __slots__ for core classes.

Because these are the classes that get instantiated many times while
iterating transactions, the performance benefit of defining __slots__
is worth the development overhead.

conservancy_beancount/data.py

@@ -62,6 +62,8 @@ class Account(str):
     or Income:Donations. This class provides additional methods for common
     account name parsing and queries.
     """
+    __slots__ = ()
+
     SEP = bc_account.sep
 
     def is_checking(self) -> bool:
@@ -115,6 +117,7 @@ class Metadata(MutableMapping[MetaKey, MetaValue]):
     This class wraps a Beancount metadata dictionary with additional methods
     for common parsing and query tasks.
     """
+    __slots__ = ('meta',)
 
     def __init__(self, source: MutableMapping[MetaKey, MetaValue]) -> None:
         self.meta = source
@@ -165,6 +168,7 @@ class PostingMeta(Metadata):
     Under the hood, this class does a little extra work to avoid creating
     posting metadata if it doesn't have to.
     """
+    __slots__ = ('txn', 'index', 'post')
 
     def __init__(self,
                  txn: Transaction,
@@ -207,6 +211,7 @@ class Posting(BasePosting):
     * The `account` field is an Account object
     * The `meta` field is a PostingMeta object
     """
+    __slots__ = ()
 
     account: Account
     # mypy correctly complains that our MutableMapping is not compatible