lang: for-loop over List(T); deref a *T method receiver

The collection for-loop now iterates a List(T)-like struct ({ items: [*]T, len, … }) — and a *List — by viewing it as items[0..len]. So 'for legal: (m)' / 'for pieces: (*p)' work like iterating a slice, with by-ref captures writing back into the backing.

fixupMethodReceiver also derefs a *T receiver when the method takes T by value, so a 'for xs: (*x)' capture can call value-self methods (x.method()). Regression: examples/for-list.sx.

tests/expected/for-list.exit

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

tests/expected/for-list.txt

@@ -0,0 +1,4 @@
+sum 60
+sum2 360
+via ptr 360
+boxes 7