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, 17 insertions, 2 deletions

diff --git a/pkg/ast/testdata/all.txt b/pkg/ast/testdata/all.txt
index 268b49a47..d4452b34f 100644
--- a/pkg/ast/testdata/all.txt
+++ b/pkg/ast/testdata/all.txt
@@ -48,5 +48,20 @@ s2 {
 
 type mybool8 int8
 type net_port proc[1, 2, int16be]
-type mybool16				### unexpected '\n', expecting int, identifier, string
-type type4:4 int32			### unexpected ':', expecting int, identifier, string
+type mybool16				### unexpected '\n', expecting '[', identifier
+type type4:4 int32			### unexpected ':', expecting '[', identifier
+
+type templ0[] int8			### unexpected ']', expecting identifier
+type templ1[A,] int8			### unexpected ']', expecting identifier
+type templ2[,] int8			### unexpected ',', expecting identifier
+type templ3[				### unexpected '\n', expecting identifier
+type templ4[A]				### unexpected '\n', expecting int, identifier, string
+type templ5[A] const[A]
+type templ6[A, B] const[A, B]
+type templ7[0] ptr[in, int8]		### unexpected int, expecting identifier
+
+type templ_struct0[A, B] {
+	len	len[parent, int16]
+	typ	const[A, int16]
+	data	B
+} [align_4]