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, 4 insertions, 3 deletions

diff --git a/pkg/ast/test_util.go b/pkg/ast/test_util.go
index 0aed0a2dc..b9fe12152 100644
--- a/pkg/ast/test_util.go
+++ b/pkg/ast/test_util.go
@@ -7,6 +7,7 @@ import (
 	"bufio"
 	"bytes"
 	"io/ioutil"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -41,7 +42,7 @@ func NewErrorMatcher(t *testing.T, file string) *ErrorMatcher {
 				break
 			}
 			errors = append(errors, &errorDesc{
-				file: file,
+				file: filepath.Base(file),
 				line: i,
 				text: strings.TrimSpace(string(ln[pos+3:])),
 			})
@@ -82,13 +83,13 @@ nextErr:
 			want.matched = true
 			continue nextErr
 		}
-		t.Errorf("unexpected error: %v:%v:%v: %v", e.file, e.line, e.col, e.text)
+		t.Errorf("unexpected error:\n%v:%v:%v: %v", e.file, e.line, e.col, e.text)
 	}
 	for _, want := range em.expect {
 		if want.matched {
 			continue
 		}
-		t.Errorf("unmatched error: %v:%v: %v", want.file, want.line, want.text)
+		t.Errorf("unmatched error:\n%v:%v: %v", want.file, want.line, want.text)
 	}
 }