spec: add missing boolean rules

[zerbina]

Summary

• add reduction rules for Ident("true") and Ident("false")
• add reduction and type deduction rules for not

[zerbina]

It's also possible for Ident("true") and Ident("false") to be considered values, in which case True and False would become defs rather than meta-language values / type constructors.

Intuitively, I would say that the current approach is better in the short term, though it might have to be revisited in the longer term.