[Crystal] Allow double colons in model name

[cyangle]

Allows double colons in model names. So that cli option --type-mappings=AnyType=JSON::Any works out of the box.

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[cyangle]

@wing328 I mainly added method sanitizeModelName and use it in method toModelName.

I also added a few basic tests and formatted the code.

[cyangle]

It seems like there are test failures in the unit tests job but github actions reported success.

[wing328]

LGTM