Please introduce MarshalINI() ([]byte, error) and UnmarshalINI([]byte) error interface for custom structs

[SharkFourSix]

Describe the feature

So far nobody (not even the documentation) has shown a way to marshal data types.

Describe the solution you'd like

Introduce marshaller interfaces like so:

type INIUnmarshaller interface {
    UnmarshalINI([]byte) error
}

type INIMarshaller interface {
    MarshalINI([]byte, error)
}

Describe alternatives you've considered

Alternative: Manually load data as string and then convert into custom type

Additional context

Usage:

type Permission int
const (
    None Permission = iota
    Read Permission
    Write
    Edit
    Group
)


type Config struct {
    Foo string
    Permission Permission
}

var permToStringMap = map[string]Permission = {
    "none": None,
    "read": Read,
    "write": Write,
    "edit": Edit,
    "group": Group,
} 

func (c *Permission) UnmarshalINI(p []byte) error {
    p, ok := permToStringMap[string(p)]
    if !ok {
        return fmt.Errorf("invalid permission `%v`", p)
    }
    *c = p
    return nil
}

config.ini

permission = "Group"

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