Upgrade to Scala 3.3.1

[sankalpgambhir]

As filed in CHANGES.md:

Upgrade to Scala 3.3.1

• the Proof class has been temporarily unsealed, till dotty #19031 / lisa Unsealed Proof class #190 is fixed (not linked here to avoid spamming everyone on those issues)
• minor fixes to address compilation errors, mostly involving explicitly specifying types or implicits in some places.

[sankalpgambhir]

Oh also fixes #188 probably as I was playing with that for my exam question. This is done somewhat shadily with a merge strategy that discards duplicates whose source I am unsure of.

[sankalpgambhir]

PS: I did attempt to add @unchecked to the relevant pattern match isntead of unsealing, but that leads to the same compiler crash.