For Void and Either
This is because I ended up using them elsewhere, so why not include them in the stdlib.
Also expose left/rightInjective functions, as are used in the DecEq proofs.
This is partly to tidy things up, but also a good test for 'import as'.
Requires some internal changes since there are parts of reflection,
unelaboration and a compiler hack that rely on where things are in the
Prelude.
