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).

examples/139-expression-bodied-fn.sx

@@ -0,0 +1,36 @@
+// M1.0 — expression-bodied function declarations.
+//
+// sx's `=>` body form (already used for lambdas) extends to
+// top-level and struct-member function declarations:
+//
+//     name :: (params) -> RetType => expr;
+//
+// Pins three positions: module-top, struct method, niladic.
+
+#import "modules/std.sx";
+
+double :: (x: s32) -> s32 => x * 2;
+
+sum :: (a: s32, b: s32) -> s32 => a + b;
+
+answer :: () -> s32 => 42;
+
+Point :: struct {
+    x: s32;
+    y: s32;
+
+    total :: (self: *Point) -> s32 => self.x + self.y;
+    scaled :: (self: *Point, by: s32) -> s32 => (self.x + self.y) * by;
+}
+
+main :: () -> s32 {
+    print("double: {}\n", double(7));
+    print("sum: {}\n", sum(3, 4));
+    print("answer: {}\n", answer());
+
+    p := Point.{ x = 10, y = 20 };
+    print("total: {}\n", p.total());
+    print("scaled: {}\n", p.scaled(3));
+
+    0;
+}

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