Avoid duplicate listing of subtype name when adding a subtype

Currently, when adding a subtype its name has to be listed twice. The is a very common pattern. For example:

```
struct BFrameHeader:
0 [+ BasicL2CAPHeader.$size_in_bytes] BasicL2CAPHeader basic_l2cap_header
```

It would be great if user can just list the struct once. For example, here are some possible syntaxes for above:

```
struct BFrameHeader:
0 [+current.$size_in_bytes] BasicL2CAPHeader basic_l2cap_header
```

```
struct BFrameHeader:
0 [+*] BasicL2CAPHeader basic_l2cap_header
```