Properly support multiple namespaces in Go

[VeaaC]

namespace n{
const i8 FOO = 0;
}

namespace m{
const i8 FOO = 1;
}

Currently results in

const (
...
    FOO int8 = 0
    FOO int8 = 1
)

Similarly for other constructs. This needs to be fixed if namespaces are to be properly supported by the Go backend.

FYI: @artemnikitin