Allow multiple PrimitiveGuard annotations

[leonschoorl]

Fixes #1436

Due to some refactoring of the error reporting this slightly changes some of the error messages:

import Clash.Prelude
topEntity = False
{-# ANN topEntity (defSyn "foo") #-}
{-# ANN topEntity (defSyn "bar") #-}

Now generates:

    Error processing 'Synthesize' annotations on Main.topEntity:
    A binder can't have more than one 'Synthesize' annotation.

Instead of:

    The following value has multiple 'Synthesize' annotations: topEntity

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[martijnbastiaan]

Cool :). Tiny changes requested.