v1.0.0: Improved error handling, user-defined serialization

This is the first release under the dlang-community banner.

It features a few internal improvements, doc fixes,
as well as the following major changes since the last release (v0.19.1):
- `ensureNotInGC` will no longer allocate (#70);
- `toString(sink)` or `toString` will now be used for serialization if found on an aggregate (#68);
- Every `SqliteException` instance now has an error code (#60);
- `VariantException` will no longer be catched and wrapped in an `SqliteException` (#60);