Better function call syntax for Pancake

[IlmariReissumies]

This commit revamps the function call syntax of Pancake to make it more intuitive. Previously two kinds of function calls were accepted as declarations:

fun f() {
  var x = 5;
  g(); // tail call
  x = g(); // assigning call (formerly and confusingly known as "returning call")
  return 0
}

This syntax is confusing --- the tail call will immediately return from f() , causing the second call to g() to be never evaluated, contrary to programmer intuition.

In the new style, we allow (and distinguish for purposes of compilation) four kinds of calls:

fun f() {
  var 1 x = g(); // declaring call
  g(); //stand-alone call
  x = g(); // assigning call
  return g(); // tail call
}

Declaring calls, unlike other calls, cannot have exception handlers; if g() above raises an exception, this would leave x uninitialised. Note that declaring calls need a shape annotation on the variable, because in general the shape of the return value is not statically known. (I initially tried adding this annotation on the function declaration instead, but this incorrectly assumes it is always statically known which function is being called).

Stand-alone calls have their return values ignored, but unlike tail calls, won't cause the callee to also return.

Tail calls have the return in front of them; hopefully that way nobody will be surprised when they return :)