TypeScript definitions for future supporting libraries

[martinheidegger]

It'd be pretty important to have typescript definition files for following libraries (priority sorted)

• streamx - basis for accessing/interacting with hypercores
• hypercore - basis for storage
• abstract-encoding - part of hypercore options
• random-access-memory - To be used for tests

Order of preference for location of typescript definitions files:

• In the source repository (requires OK from maintainer)
• In the DefinitelyTyped repository.
• In the ./src/types folder of this repo.

Note: there is previous effort for this in sams-dat-api
List can be extended in future