eip1898: add unit tests to secure all conversions and impl

[tcoratger]

• Full path under serde is used instead of import under cfg flag.
• Some functions are simplified.
• Unit tests are added to protect all implementations and conversions that can be tricky to mix.

[zerosnacks]

Proposed changes are sensible and does not appear to contain breaking changes. Tests look good, ran tests against main and verified there were no regressions.

[tcoratger]

Proposed changes are sensible and does not appear to contain breaking changes. Tests look good, ran tests against main and verified there were no regressions.

@zerosnacks Yes exactly, that's exactly why I was doing this PR. My constant was that there were a lot of conversions in this file and that simplifications could be made in order to end up with exactly the same code while having a less wordy and error-prone implementation.

• So I simplified the places that could be simplified by using all the available conversions as much as possible.
• I took care to normally add all the unit tests covering all situations because these unit tests are very quick to run and they allow two things:
  • Check that this PR doesn't break anything and that everything is correct (conversions, string manipulations...)
  • Be sure that we don't break anything in the future as soon as we touch this EIP.

[zerosnacks]

cc @yash-atreya pending final review, LGTM