Add transaction coders

[AlicanC]

This PR adds a new package called "transactions" that contains all types defined in tx_format.md and coders for them.

A few notes:

• Encoding is memory intensive since all parts are kept in memory before concatenation.
• Code is a bit ugly because I didn't want to commit to an abstraction at this time. Can be cleaned up later.
• Tests cover 98% of the code but they aren't that great. They can be improved as we actually start using the code.

Resolves #15

[QuinnLee]

no blockers! there are some lint warnings, but nothing worth blocking the merge

[AlicanC]

I've just fixed the conflict on tsconfig so I need another approval @QuinnLee, thanks!