Serde 16 intergers

[Thykof]

Context
Describe / explain why we should do this: motivations, context or other info. Keep it brief and simple - PM

User flow
Describe the user flow using user stories so the end result is super clear - PM

How to
List the step-by-step to get it do if needed - PM

Add helpers in serialization/numbers.ts:

• u16toBtes
• bytesTou16

Add Args.nextU16() that use the created helpers

Technical details
Give the technical insights so anyone in the team can tackle the tasks - Dev

QA testing
Does this task require some QA tests ?
If yes, explain how to validate it

[gregLibert]

@Ben-Rey I think we should put this issue on hold until it's needed by an end user. Protobuf could solve this so I don't want to lose time here.