StringMap<T> => Map<String,T>

I found it a bit weird to use `StringMap<T>` instead of `Map<String,T>` in the std lib.
Since `Map<String,T>` allows ppl to use `map[key]` syntax, which `StringMap<T>` doesn't, `Map<String,T>` is more flexible to ppl.

Here I replaced all `StringMap<T>` presented in the public API in the std lib with `Map<String,T>`.
I also searched for `IntMap`, `EnumValueMap`, and `ObjectMap`, but they simply do not appear in the API.