Clean up `encoding` module, part 2

[kt3k]

encoding module has the following encodings and corresponding decode and encode methods:

• ascii85
  • encode: Uint8Array -> string
  • decode: string -> Uint8Array
• base32
  • encode: Uint8Array -> string
  • decode: string -> Uint8Array
• base58
  • encode: ArrayBuffer | string -> string
  • decode: string -> Uint8Array
• base64
  • encode: ArrayBuffer | string -> string
  • decode: string -> Uint8Array
• base64url
  • encode: ArrayBuffer | string -> string
  • decode: string -> Uint8Array
• hex
  • encode: Uint8Array -> Uint8Array
  • decode: Uint8Array -> Uint8Array
• varint
  • encode: bigint | number -> [Uint8Array, number]
  • decode: Uint8Array -> [bigint, number]

Encodings except varint are similar in typing, but there are discrepancies (The case of hex is pointed in #3605). I think these 6 encodings should have the same typings, for example encode: Uint8Array | ArrayBuffer | string -> string; decode: string -> Uint8Array

part of #3489

[lino-levan]

@kt3k is this completed?

[kt3k]

@lino-levan Yeah, let's close this. Thanks for your contributions!