Add support to read Hash as Map when calling load()

[NuriYuri]

To not break retro-compatibility we can add an option stating how Hash should be treated:

1. legacy will be loaded as a JS object (default)
2. mapWhenBadKeys will be loaded as a JS object if keys are all string or symbol, otherwise loaded as Map
3. alwaysMap will be loaded as Map

There could be a global option & local option for Hash loading so it's not annoying to specify it.