Fix Marshaler support for slices of interfaces.

[grahammiln]

Support encoding of general interfaces slices,[]interface{}, and slices of constrained interfaces.

[DHowett]

Thanks! As with your prior PR (for which I'm also grateful) I'll need to get to this in a bit. I'm hoping to have some time this week. :)

[DHowett]

I think this may have been fixed by a3baacb!

If I back out your changes in marshal.go, the tests still pass (and produce intact property lists:)

=== RUN   TestInterfaceSliceMarshal
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0"><array><dict><key>Name</key><string>dog</string></dict></array></plist>
--- PASS: TestInterfaceSliceMarshal (0.00s)

=== RUN   TestInterfaceGeneralSliceMarshal
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0"><array><dict><key>Name</key><string>dog</string></dict><string>a string</string><integer>1</integer><true/></array></plist>
--- PASS: TestInterfaceGeneralSliceMarshal (0.00s)

[DHowett]

Thanks for the contributions, of course. Sorry to take ages to get back to you. :)