Make use of new features in Agda 2.6.0

[gallais]

We could either wait for it to be released or resurrect experimental.

• Check the library with the --safe flag on
• Use primEraseEquality (in Agda.Builtin.Equality.Erase)
• Use generalisation
• Use smarter coverage checker
• The warnings for missing fixities should now work properly

[MatthewDaggitt]

Given the problems we've had in the past, my gut feeling is that we should hold off for a while until 2.6.0 has a more firm release date? Unless I've missed something and it's coming out soon?

[UlfNorell]

We're aiming for a Christmas release, but as usual, no promises.

[MatthewDaggitt]

Hmm in that case we probably should get on it. Feel free to bring the experimental branch up-to-date.

[MatthewDaggitt]

@gallais what does the last point on the list refer to?

[gallais]

It refers to #207

[MatthewDaggitt]

It refers to #207

Okay thanks. Closing this as it has its own issue.