[ new ] Agda.Utils.Lens: Kmett style lenses.

Rather than depending on the monstrous lens package, we pick out the
most useful lens operations.  We keep compatibility with the lens
package though, by using the original identifiers.
[ refactor ] Kmett-style lens for loneSigs.
[ cosmetics ] Use Map instead of Map.Map.
[ refactored ] and documented plusFixities.
[ refactored ] fixities and added missing clause for InstanceB.

Use a foldMap instead of hand-rolled recursion.
[ new ] Agda.Utils.Functor.(<&>) as flip (<$>).

Neat for defining Kmett/van Larhoven lenses.
[ test ] for unbound fixity declaration in instance block.
[ refactored ] fixities again: expanded catch-all.
[ clean ] Removed old commented-out code.
[ haddock ] Purpose statement for module Agda.Syntax.Concrete.Definitions.
[ Issue 1204 ] Fixed: better error message.
[ Issue 1204 ] Test case and updated error messages.