* remove common/constants (all in models now)
* remove common/txFlags (all in models now)
* move ecdsa from src/common to src
* move errors from src/common to src, export at top level
* sets up linting config and runs `yarn lint --fix` once, so that all changes will show up correctly in future PRs.
* Note that there are still a lot of linter errors.
- Defines a TypeScript type for PaymentTransaction
- Provides an optional function to users for verifying a PaymentTransaction instance at runtime: verifyPaymentTransaction()
- Adds tests for verifyPaymentTransaction()
- Adds isFlagEnabled() util to be used for models
