Renaming of partially decorated references to unique references

[krame505]

Changes

• "Partially decorated references" are now referred to as "unique references"
• The type expression syntax PartiallyDecorated is changed to Decorated! (suggested by @RandomActsOfGrammar)

Documentation

This will be reflected in the updated docs once feature/uniqueness-analysis is ready to merge.

[ericvanwyk]

Can you indicate the motivation for Decorated!? Does this come from some other language or system?

[krame505]

According to @RandomActsOfGrammar:

In logic, sometimes people write ∃! to mean "there exists a unique item such that". That's why I suggested it. I don't know if any other software systems use it.

[ericvanwyk]

Something like this should be part of the documentation. Maybe find out more what systems/logics do this. Thanks.