Swift: add unit tests to code generation

Tests can be run with
```
bazel test //swift/codegen:tests
```

Coverage can be checked installing `pytest-cov` and running
```
pytest --cov=swift/codegen swift/codegen/test
```

swift/codegen/lib/ql.py

@@ -0,0 +1,88 @@
+import pathlib
+from dataclasses import dataclass, field
+from typing import List, ClassVar
+
+import inflection
+
+
+@dataclass
+class QlParam:
+    param: str
+    type: str = None
+    first: bool = False
+
+
+@dataclass
+class QlProperty:
+    singular: str
+    type: str
+    tablename: str
+    tableparams: List[QlParam]
+    plural: str = None
+    params: List[QlParam] = field(default_factory=list)
+    first: bool = False
+    local_var: str = "x"
+
+    def __post_init__(self):
+        if self.params:
+            self.params[0].first = True
+        while self.local_var in (p.param for p in self.params):
+            self.local_var += "_"
+        assert self.tableparams
+        if self.type_is_class:
+            self.tableparams = [x if x != "result" else self.local_var for x in self.tableparams]
+        self.tableparams = [QlParam(x) for x in self.tableparams]
+        self.tableparams[0].first = True
+
+    @property
+    def indefinite_article(self):
+        if self.plural:
+            return "An" if self.singular[0] in "AEIO" else "A"
+
+    @property
+    def type_is_class(self):
+        return self.type[0].isupper()
+
+
+@dataclass
+class QlClass:
+    template: ClassVar = 'ql_class'
+
+    name: str
+    bases: List[str] = field(default_factory=list)
+    final: bool = False
+    properties: List[QlProperty] = field(default_factory=list)
+    dir: pathlib.Path = pathlib.Path()
+    imports: List[str] = field(default_factory=list)
+
+    def __post_init__(self):
+        self.bases = sorted(self.bases)
+        if self.properties:
+            self.properties[0].first = True
+
+    @property
+    def db_id(self):
+        return "@" + inflection.underscore(self.name)
+
+    @property
+    def root(self):
+        return not self.bases
+
+    @property
+    def path(self):
+        return self.dir / self.name
+
+
+@dataclass
+class QlStub:
+    template: ClassVar = 'ql_stub'
+
+    name: str
+    base_import: str
+
+
+@dataclass
+class QlImportList:
+    template: ClassVar = 'ql_imports'
+
+    imports: List[str] = field(default_factory=list)