python: Implement phogen encode

Implement phogen.encode() to encode Bhashes to phogen strings. This
commit also adds unit tests for the above function.

tests/meson.build

@@ -9,8 +9,11 @@ test('test_phogen_map',
            'PHOGEN_MAP_PY=@0@'.format(_phogen_map_py),
            'PHOGEN_WORD_LIST=@0@'.format(_word_list)])
 
-test('test_bhash',
+test('test_bhash_py',
     find_program('test_bhash.py'),
-    env: ['SOURCE_ROOT=' + meson.project_source_root(),
-          'PYTHONPATH=' + PYTHONPATH])
+    env: ['PYTHONPATH=' + PYTHONPATH])
+
+test('test_phogen_py',
+    find_program('test_phogen.py'),
+    env: ['PYTHONPATH=' + PYTHONPATH])