Upgrade to Kotlin 1.4

Dev/0.4.3 (#7)

* upgrade to kotlin 1.4 and serialization 1.0.0-RC

* exposed serialization core as an API since the platform specific libs were removed

Even Better Commission Calculation

Slightly improved commission calculation and error handling in case no funds are spendable

Better Commission Calculation

Commission estimation is more robust now
Fixed asset payments by adding support for change outputs

Data feed

Added support for data feed messages

Composing Payments

This release add supports for

• composing and posting byte and asset payments
• retrieving balances
• subscribing to an address to get updates
• minimal single address wallets

Added get_joint

Well, this release is functionally the same as 0.2.0, only GitHub CI scripts has changed
Library version wasn't updated to 0.2.1 so it didn't get published to the maven repo