Always use Map for int64 keys

```
message Foo {
  map<int64, int64> bar = 1;
}
```

is generated to

```ts
export interface Foo {
  bar: { [key: Long]: Long }
}
```

when `--ts_proto_opt=forceLong=long`

This is a type error:

```
An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
```