Default to using kielExtensions and deprecate --extended

Fixes #137 (closed) by

• enabling kielExtensions (FunctionalPatterns, AnonFreeVars) by default
• deprecating --extended
• adding the negated extensions (NoFunctionalPatterns, NoAnonFreeVars), which take precedence over the positive variants and are removed in the ExtensionCheck

The ExtensionCheck now uses a separate 'phase' to apply all implicit extensions. This also fixes a bug where e.g. -XNoImplicitPrelude would not automatically enable -XNoDataDeriving since this transformation would only be applied on pragmas.

In the medium-term we might want to consider representing negated extensions in a different way to avoid redundancies. For example, we could allow every extension to be negated and internally only track the set of enabled extensions.