Remove class initialization, use global cache

[mpetrunic]

          Why do we need this `new Sprinter` intialization?

Wouldn't it be more intuitive and dev friendly to do:

import {getUserBalances} from "@chainsafe/sprinter-sdk"

balance = await getUserBalances(address, optionalConfigWithBaseUrl)

?

Originally posted by @mpetrunic in #61 (comment)

• remove class initialization, somewhat function approach
• use global cache to cache tokens, chains and routes