More classical if syntax

[Champii]

The actual if syntax is nice for compact functions but does not blend nicely into more complex design

We want a more classical syntax like

main: ->
  if true
    print a
  else
    print "no"

[Champii]

We also want to accept onliners

main: -> if true then print 1 else print "no"

[Champii]

Classical if is implemented in #78