panic with value res composite types

@jordansissel reports in https://github.com/purpleidea/mgmt/discussions/977 I haven't tested this yet, but it's likely this is our bug. Either we need to catch composite types and disallow with panic, or handle them correctly. ``` $example = struct { foo => "Hello world", } value "example" { any => $example, Meta:export => [ "*" ], } ``` This errors: ``` 23:21:47 main: waiting... 23:21:47 gapi: lang: stream... 23:21:47 gapi: lang: function engine starting... 23:21:47 gapi: lang: interpret: interpreting... panic: struct has unexported field: foo goroutine 354 [running]: github.com/purpleidea/mgmt/lang/types.(*Type).Reflect(0xc001283590) github.com/purpleidea/mgmt/lang/types/type.go:1076 +0xc85 github.com/purpleidea/mgmt/lang/types.(*StructValue).Value(0xc0012925d0) github.com/purpleidea/mgmt/lang/types/value.go:1413 +0x5b github.com/purpleidea/mgmt/lang/types.Into({0x3d0c588, 0xc0012925d0}, {0x33967e0?, 0xc000373360?, 0x3?}) ``` This also panics even without exporting (no Meta:export => ...)