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.

src/libutil/file-content-address.hh

@@ -23,6 +23,23 @@ enum struct FileIngestionMethod : uint8_t {
     Recursive = 1,
 };
 
+/**
+ * Parse a `FileIngestionMethod` by name. Choice of:
+ *
+ *  - `flat`: `FileIngestionMethod::Flat`
+ *  - `nar`: `FileIngestionMethod::Recursive`
+ *
+ * Oppostite of `renderFileIngestionMethod`.
+ */
+FileIngestionMethod parseFileIngestionMethod(std::string_view input);
+
+/**
+ * Render a `FileIngestionMethod` by name.
+ *
+ * Oppostite of `parseFileIngestionMethod`.
+ */
+std::string_view renderFileIngestionMethod(FileIngestionMethod method);
+
 /**
  * Dump a serialization of the given file system object.
  */