Add reusable generic RIFF types

There are two types of RIFF structs: with asserts (chunk and
parent_chunk_data) and without asserts (chunk_generic and
parent_chunk_data_generic). Those without asserts won't be
needed when kaitai-io/kaitai_struct#435 (comment) is resolved.

common/riff/chunk.ksy

@@ -0,0 +1,21 @@
+meta:
+  id: chunk
+  encoding: ASCII
+  endian: le
+params:
+  - id: expected_id
+    type: str
+seq:
+  - id: id_assert
+    type: str
+    size: 4
+    valid: expected_id
+  - id: len
+    type: u4
+  - id: data_slot
+    type: slot
+    size: len
+  - id: pad_byte
+    size: len % 2 # if size is odd, there is 1 padding byte
+types:
+  slot: {} # Keeps _io for later use of same substream

common/riff/chunk_generic.ksy

@@ -0,0 +1,17 @@
+meta:
+  id: chunk_generic
+  encoding: ASCII
+  endian: le
+seq:
+  - id: id
+    type: str
+    size: 4
+  - id: len
+    type: u4
+  - id: data_slot
+    type: slot
+    size: len
+  - id: pad_byte
+    size: len % 2 # if size is odd, there is 1 padding byte
+types:
+  slot: {} # Keeps _io for later use of same substream

common/riff/parent_chunk_data.ksy

@@ -0,0 +1,17 @@
+meta:
+  id: parent_chunk_data
+  encoding: ASCII
+  endian: le
+params:
+  - id: expected_form_type
+    type: str
+seq:
+  - id: form_type_assert
+    type: str
+    size: 4
+    valid: expected_form_type
+  - id: subchunks_slot
+    type: slot
+    size-eos: true
+types:
+  slot: {} # Keeps _io for later use of same substream

common/riff/parent_chunk_data_generic.ksy

@@ -0,0 +1,13 @@
+meta:
+  id: parent_chunk_data_generic
+  encoding: ASCII
+  endian: le
+seq:
+  - id: form_type
+    type: str
+    size: 4
+  - id: subchunks_slot
+    type: slot
+    size-eos: true
+types:
+  slot: {} # Keeps _io for later use of same substream