pkg/compiler: deduplicate Types in descriptions

Add prog.Ref Type that serves as a proxy for real types
and allows to deduplicate Types in generated descriptions.
The Ref type is effectively an index in an array of types.
Just before serialization pkg/compiler replaces real types
with the Ref types and prepares corresponding array of real types.
When a Target is registered in prog package, we do the opposite
operation and replace Ref's with the corresponding real types.

This brings improvements across the board:
compiler memory consumption is reduced by 15%,
test building time by 25%, descriptions size by 33%.

Before:
$ du -h sys/linux/gen
54M	sys/linux/gen

$ time GOMAXPROCS=1 go test -p=1 -c ./prog
real	0m54.200s
real	0m53.883s

$ time GOMAXPROCS=1 go install -p=1 ./tools/syz-execprog
real	0m27.911s
real	0m27.767s

$ TIME="%e %P %M" GOMAXPROCS=1 time go tool compile ./sys/linux/gen
20.59 100% 3200016
20.97 100% 3445976
20.25 100% 3209684

After:
$ du -h sys/linux/gen
36M	sys/linux/gen

$ time GOMAXPROCS=1 go test -p=1 -c ./prog
real	0m42.290s
real	0m43.230s

$ time GOMAXPROCS=1 go install -p=1 ./tools/syz-execprog
real	0m24.337s
real	0m24.727s

$ TIME="%e %P %M" GOMAXPROCS=1 time go tool compile ./sys/linux/gen
19.11 100% 2764952
19.66 100% 2787624
19.35 100% 2749376

Update #1580

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/syz-check/check.go b/tools/syz-check/check.go
index 8cfc20294..76a52c50d 100644
--- a/tools/syz-check/check.go
+++ b/tools/syz-check/check.go
@@ -351,7 +351,7 @@ func parseDescriptions(OS, arch string) ([]*prog.KeyedStruct, map[string]*ast.St
 	if prg == nil {
 		return nil, nil, nil, fmt.Errorf("failed to compile descriptions:\n%s", errorBuf.Bytes())
 	}
-	prog.RestoreLinks(prg.Syscalls, prg.Resources, prg.StructDescs)
+	prog.RestoreLinks(prg.Syscalls, prg.Resources, prg.StructDescs, prg.Types)
 	locs := make(map[string]*ast.Struct)
 	for _, decl := range top.Nodes {
 		switch n := decl.(type) {