Proper parse and render functions for FileIngestionMethod and ContentAddressMethod

No outward facing behavior is changed.

Older methods with same names that operate on on method + algo pair (for
old-style `<method>:algo`) are renamed to `*WithAlgo`.)

The functions are unit-tested in the same way the names for the hash
algorithms are tested.

tests/unit/libstore/content-address.cc

@@ -0,0 +1,35 @@
+#include <gtest/gtest.h>
+
+#include "content-address.hh"
+
+namespace nix {
+
+/* ----------------------------------------------------------------------------
+ * ContentAddressMethod::parse, ContentAddressMethod::render
+ * --------------------------------------------------------------------------*/
+
+TEST(ContentAddressMethod, testRoundTripPrintParse_1) {
+    for (const ContentAddressMethod & cam : {
+        ContentAddressMethod { TextIngestionMethod {} },
+        ContentAddressMethod { FileIngestionMethod::Flat },
+        ContentAddressMethod { FileIngestionMethod::Recursive },
+    }) {
+        EXPECT_EQ(ContentAddressMethod::parse(cam.render()), cam);
+    }
+}
+
+TEST(ContentAddressMethod, testRoundTripPrintParse_2) {
+    for (const std::string_view camS : {
+        "text",
+        "flat",
+        "nar",
+    }) {
+        EXPECT_EQ(ContentAddressMethod::parse(camS).render(), camS);
+    }
+}
+
+TEST(ContentAddressMethod, testParseContentAddressMethodOptException) {
+    EXPECT_THROW(ContentAddressMethod::parse("narwhal"), UsageError);
+}
+
+}