Cleanup browser version of HexEncoder

[jimhark]

Proposed

HexEncoder parse() and stringify() could be freshened up.

• parse()
  • simplify var names
  • Minor reformatting
  • Throw Error() object, not string
• stringify()
  • Pad hex using .padStart(2, "0")
  • use .map().join("") to build hex string.

Alternatives Considered

Could leave as is, this is just a freshen up.

Note

PR is being prepaired.