bindings/crypto-nodejs: Several functions take/return `string` instead of a usable type

[turt2live]

For example, it'd be really great to be able to supply a JS object to encryptRoomEvent() or better understand what receiveSyncChanges is actually returning.

[turt2live]

(non-blocker for bindings usage)

[turt2live]

this also affects requests where it wants the caller to parse a string and supply a string back: surely these should be objects.