feat: add support for named output variables (#113)

* feat: add support for named output variables

Function output parameters are located at the start of the function
frame, uninitialized, as they could be only set through return
statements.

Supporting named output variables requires to:

- identify and allocate symbols corresponding to output names:
  done at ident parsing, with the funcRet() helper,

- compute the location of output name in the frame:
  done with retRank() helper,

- initialize the frame entry with the zero value of symbol type,
  as the symbol can be accessed and written at multiple times,
  and return not setting the variable (opposite to unnamed).
  Done with frameType() helper, which now takes into account
  output parameters.

* refactor: simplify memory management

Perform function frame analysis at pre-order, instead of post-order, to
initialize memory frame, and track value types. Remove all operation
involving unitialized types. Frame memory layout is now build
incrementally, instead of having to perform dedicated tree walks on AST.

_test/variadic3.go

@@ -0,0 +1,20 @@
+package main
+
+import "fmt"
+
+func f(a ...int) int {
+	fmt.Println(a)
+	res := 0
+	for _, v := range a {
+		res += v
+	}
+	return res
+}
+
+func main() {
+	fmt.Println(f(1, 2, 3, 4))
+}
+
+// Output:
+// [1 2 3 4]
+// 10