Bugfix/serializations

[chuck-h]

This PR primarily backports serialization/deserialization fixes for a few data types, specifically symbolcode, bool, and public/private keys.

It also addresses most linter complaints.

This passes the unit tests without null-check failures. However the null safety treatment is embarrassingly ad hoc and deserves a refactor.

Backported from https://github.com/JoinSEEDS/seeds_light_wallet/tree/rainbow_apk/lib/crypto/eosdart/src

[fzxu]

just double check, all tested, right?

[chuck-h]

I ran eosdart_test.dart but hadn't realized there's a whole suite of more unit tests under models dir.

Better wait until I've run those.

[chuck-h]

All tests pass. Also added my key serialization unit test to eosdart_test.dart

[fzxu]

Thanks for contribution! merged