Better type for Registers

[mkermani144]

It seems that Registers type is not narrow enough:

ergo-sdk-js/src/entities/registers.ts

Line 31 in dbdd341

export type Registers = {[key: string]: HexString}

The user may provide any string value as key, which is not correct.

It can be fixed easily with a mapped type:

- export type Registers = {[key: string]: HexString} 
+ export type Registers = {[Key in RegisterId]: HexString} 

[mkermani144]

Duplicate of #26. I don't know why two same issues were submitted, maybe a network issue.