bug: using struct-types with Capture interface is broken

In setField nodes.go:592 the type is asserted to a string to assemble the slice parameter for the Capture method. That is wrong according to the following test:

```go
type Boxes struct {
Pos lexer.Position
Boxes Box `@@`
}

type Box struct{
Pos lexer.Position
Val string `@Ident`
}

func (b *Box) Capture(values []string) error {
b.Val = values[0]
return nil
}

func TestBoxedCapture(t *testing.T) {
lex := stateful.MustSimple([]stateful.Rule{
{"Ident", `[a-zA-Z](\w|\.|/|:|-)*`, nil},
{"whitespace", `\s+`, nil},
})

parser := participle.MustBuild(&Boxes{},
participle.Lexer(lex),
participle.UseLookahead(2),
)

boxed:=&Boxes{}
if err:=parser.ParseString("test", "abc::cdef.abc", boxed);err!=nil{
t.Fatal(err)
}
}
```

This causes a type assertion panic:
```
panic: interface conversion: interface {} is parser.Box, not string [recovered]
panic: interface conversion: interface {} is parser.Box, not string

goroutine 6 [running]:
testing.tRunner.func1.2(0x1166b80, 0xc00006c810)
/usr/local/go/src/testing/testing.go:1144 +0x332
testing.tRunner.func1(0xc000001380)
/usr/local/go/src/testing/testing.go:1147 +0x4b6
panic(0x1166b80, 0xc00006c810)
/usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/alecthomas/participle/v2.setField(0xc000022880, 0x1, 0x1, 0x116f320, 0xc0000585a0, 0x199, 0x114c743, 0x5, 0x0, 0x0, ...)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/nodes.go:582 +0x23e5
github.com/alecthomas/participle/v2.(*parseContext).Apply(0xc0000102d0, 0xc000022880, 0x1)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/context.go:53 +0x150
github.com/alecthomas/participle/v2.(*strct).Parse(0xc000114d20, 0xc0000102d0, 0x116f320, 0xc0000584e0, 0x199, 0x199, 0xc00007fcc8, 0x113635e, 0x1168200, 0xc00006c4b0)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/nodes.go:121 +0x5cc
github.com/alecthomas/participle/v2.(*Parser).parseInto(0xc000114c40, 0xc0000102d0, 0x1156840, 0xc0000584e0, 0x16, 0x11c23c8, 0x128e940)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:239 +0xd0
github.com/alecthomas/participle/v2.(*Parser).parseOne(0xc000114c40, 0xc0000102d0, 0x1156840, 0xc0000584e0, 0x16, 0xc000022800, 0x1)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:222 +0x77
github.com/alecthomas/participle/v2.(*Parser).ParseFromLexer(0xc000114c40, 0xc000114e70, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:152 +0x4c5
github.com/alecthomas/participle/v2.(*Parser).parse(0xc000114c40, 0x11c1a28, 0xc000058540, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, 0x0, 0xc0000584e0)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:160 +0xe5
github.com/alecthomas/participle/v2.(*Parser).ParseString(0xc000114c40, 0x118ff92, 0x4, 0x11918f3, 0xd, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, ...)
/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:189 +0xf5
github.com/golangee/architecture/adl/parser.TestBoxedCapture(0xc000001380)
/Users/tschinke/git/github.com/golangee/architecture.git/adl/parser/parser_test.go:73 +0x1fa
testing.tRunner(0xc000001380, 0x119ad10)
/usr/local/go/src/testing/testing.go:1194 +0xef
created by testing.(*T).Run
/usr/local/go/src/testing/testing.go:1239 +0x2b3

```

btw, just closing #139 "as incorrect" was a bit offensive.