[nit] Minor improvements to Object::new_coin

[lxfind]

Description

This PR improves a few styles for clarity:

1. new_coin API should take the type_tag of the coin directly, instead of allowing arbitrary move types.
2. the implementation of new_coin API should use Coin instead of GasCoin, since it's not guaranteed to be a GasCoin.
3. Some other type improvements

Test plan

CI

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Release notes

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