TypeScript definitions for future supporting libraries

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

- [x] [streamx](https://github.com/mafintosh/streamx) - basis for accessing/interacting with hypercores
- [ ] [hypercore](https://github.com/hypercore-protocol/hypercore) - basis for storage
- [x] [abstract-encoding](https://github.com/mafintosh/abstract-encoding) - part of hypercore options
- [ ] [random-access-memory](https://github.com/random-access-storage/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](https://github.com/DefinitelyTyped/DefinitelyTyped) repository.
- In the `./src/types` folder of this repo.

Note: there is previous effort for this in [sams-dat-api](https://github.com/sammacbeth/sams-dat-api/tree/master/packages/types/lib)
_List can be extended in future_

Review the unchecked hypercore and random-access-memory entries, the preferred locations for definitions, and the previous effort in sams-dat-api. Confirm the relevant library APIs and maintainer or DefinitelyTyped requirements first. Done means usable TypeScript definitions are provided in an agreed location for both libraries.

Written by the indexing model from the issue text.