Make constructors explicit in core and ff.

[Jonathan Immanuel Brachthäuser]

Constructor calls are currently represented by PureApp nodes in the core tree and following representations.

This would be a bit more symmetrical to how the introduction and elimination of codata/objects (new) is treated.

Note that optimizations (case-of-known-case etc.) are currently possible without making constructors explicit, since we can just compare the symbols.

This works, since free variables in core (such as data types and constructors) are not renamed (which is the case for bound local variables) and the PureApp and the Match thus still point to the constructor symbol.