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]])
```

1 files changed, 9 insertions, 1 deletions

diff --git a/pkg/ast/walk.go b/pkg/ast/walk.go
index fd5065013..fe9112578 100644
--- a/pkg/ast/walk.go
+++ b/pkg/ast/walk.go
@@ -49,7 +49,15 @@ func (n *Resource) Walk(cb func(Node)) {
 
 func (n *TypeDef) Walk(cb func(Node)) {
 	cb(n.Name)
-	cb(n.Type)
+	for _, a := range n.Args {
+		cb(a)
+	}
+	if n.Type != nil {
+		cb(n.Type)
+	}
+	if n.Struct != nil {
+		cb(n.Struct)
+	}
 }
 
 func (n *Call) Walk(cb func(Node)) {