CIP-0121? | Integer-ByteString conversions (#624)

* Initial draft

* Final draft of CIP

* Redraft to use Ilia's proposal, clarification, examples

* Discussion link in CIP

* Relabel CIP

* Add another builtinByteStringToInteger property, fix incorrect property specification

* Allow empty ByteString conversions, rename padding argument to length argument

* Typo

* Better properties, fix typoes

* Fix error in representation example

* Typoes, address feedback

* apply CIP number 121 also in CIP repo README

@kozross this is likely to cause a merge conflict with the CIP repo's `master` branch; if so, just put your new CIP `121` after the merged CIP `120` and keep the more current modification date.

* Now CIP 121

* Fix merge conflict

Co-authored-by: Robert Phair <rphair@cosd.com>

* Fix heading error

Co-authored-by: Ryan <44342099+Ryun1@users.noreply.github.com>

* Remove line for CIP-121 from README

* Fix timestamp on CIP updates

Co-authored-by: Robert Phair <rphair@cosd.com>

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Co-authored-by: Robert Phair <rphair@cosd.com>
Co-authored-by: Ryan <44342099+Ryun1@users.noreply.github.com>