pkg/compiler: support type templates

Netlink descriptions contain tons of code duplication,
and need much more for proper descriptions. Introduce
type templates to simplify writing such descriptions
and remove code duplication.

Note: type templates are experimental, have poor error handling
and are subject to change.

Type templates can be declared as follows:

```
type buffer[DIR] ptr[DIR, array[int8]]
type fileoff[BASE] BASE
type nlattr[TYPE, PAYLOAD] {
	nla_len		len[parent, int16]
	nla_type	const[TYPE, int16]
	payload		PAYLOAD
} [align_4]
```

and later used as follows:

```
syscall(a buffer[in], b fileoff[int64], c ptr[in, nlattr[FOO, int32]])
```

0 files changed, 0 insertions, 0 deletions