feat: make tickers module keep token exponents too

[hallazzang]

Description

Instead of simply keeping denom-ticker mappings, this PR introduces new Asset type which holds the exponent as well. With this change it's possible to calculate dollar values of an asset correctly with assets with different exponents.

Closes: MILK-66

See #78 for previous discussions.

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[hallazzang]

Sorry for confusion, closing this since I force-pushed cherry-picked commits to #78 and reopened it.