FAISS and embedding support (#48)

also adds embeddings and an in memory docstore

tests/unit_tests/docstore/test_inmemory.py

@@ -0,0 +1,21 @@
+"""Test in memory docstore."""
+
+from langchain.docstore.document import Document
+from langchain.docstore.in_memory import InMemoryDocstore
+
+
+def test_document_found() -> None:
+    """Test document found."""
+    _dict = {"foo": Document(page_content="bar")}
+    docstore = InMemoryDocstore(_dict)
+    output = docstore.search("foo")
+    assert isinstance(output, Document)
+    assert output.page_content == "bar"
+
+
+def test_document_not_found() -> None:
+    """Test when document is not found."""
+    _dict = {"foo": Document(page_content="bar")}
+    docstore = InMemoryDocstore(_dict)
+    output = docstore.search("bar")
+    assert output == "ID bar not found."