Feat: give output as json from abi-based methods

[birchmd]

This allows nice output when calling from the NEAR CLI. Example:

$ near --networkId testnet --accountId birchmd.testnet call dev-1663014663747-27418521013742 multiply '{"a": 7, "b": 6}'

Scheduling a call: dev-1663014663747-27418521013742.multiply({"a": 7, "b": 6})
Doing account.functionCall()
Transaction Id 97g3DMfcZMyipJ5QLzRw46d6SvnjsTdyn1B1ZVH5w7fg
To see the transaction in the transaction explorer, please open this url in your browser
https://explorer.testnet.near.org/transactions/97g3DMfcZMyipJ5QLzRw46d6SvnjsTdyn1B1ZVH5w7fg
{ output: 42, status: 'SUCCESS' }

This feature is implemented by having environment functions like value_return and revert only update the internal EVM state (setting the exit status and return data), not call any external exit methods (like process::exit, or NEAR's host functions). Instead, the exit calls happen as part of a new function called post_exec. This gives a chance for the return data to be re-encoded from ABI into JSON before the value is returned.

The main files to focus on in the review are: compiler.rs, api.rs and encode.rs. Note that the code for decoding JSON into ABI input was simply moved from api.rs to decode.rs with no changes.