ffi M1.0 (1/3): lock in expression-bodied top-level + struct-method form

sx's '=>' body form (already used for lambdas) works today for
top-level function declarations and struct member methods. Pin
the surface with examples/139-expression-bodied-fn.sx so a
parser regression here surfaces immediately.

Coverage:
- module-top:      double :: (x: s32) -> s32 => x * 2;
- niladic:         answer :: () -> s32 => 42;
- struct method:   total :: (self: *Point) -> s32 => self.x + self.y;

Next: extend the same form to '#objc_class' member methods (the
M2.1(a/b) class-constant + class-method overrides path).

tests/expected/139-expression-bodied-fn.exit

@@ -0,0 +1 @@
+0

tests/expected/139-expression-bodied-fn.txt

@@ -0,0 +1,5 @@
+double: 14
+sum: 7
+answer: 42
+total: 30
+scaled: 90