Patrick McAndrew

add missing kStringMaxLength, constants.MAX_LENGTH & constants.MAX_STRING_LENGTH fixes #311

There appears to be a missing implementation of `buffer.constants.MAX_LENGTH` & `buffer.constants.MAX_STRING_LENGTH` that is documented in the [node buffer api](https://nodejs.org/docs/latest-v14.x/api/buffer.html#buffer_buffer_constants)

The current Iso 4217 data includes SLE which is currently missing. This PR adds this to the list. See https://en.wikipedia.org/wiki/ISO_4217 ## Checklist - [x] PR contains only changes related; no...