Add if-then-else statement support

[stringhandler]

Built on #185, so that would need to be merged first.

Addresses #181

[gerau]

Could we make the else expression optional? Maybe we could evaluate a missing else as the unit type (), so this would prevent conflicts when using an if statement in an assignment, for example.

[KyrylR]

#185 is merged, so you can rebase

[uncomputable]

This is an if-then-else expression, by the way.