lang migration: rename signed integer types sN -> iN

Mechanical sweep of all .sx sources, plan docs, and tests/expected
snapshots for the sx language rename (s8/s16/s32/s64 -> i8/i16/i32/i64).
Verified: tools/run_tests.sh 23/23.

Note: the ios-sim build has 2 pre-existing 'restart' dot-call errors
from the sx opt-in UFCS change (sx a47ea14) — independent of this
rename (present pre-sweep); migrated in the follow-up commit.

tests/swap_legality.sx

@@ -44,11 +44,11 @@ boards_equal :: (x: *Board, y: *Board) -> bool {
     true
 }
 
-cell :: (col: s64, row: s64) -> Cell {
+cell :: (col: i64, row: i64) -> Cell {
     Cell.{ col = col, row = row }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== swap & legality ==\n");
 
     // Board whose ONLY swap-formable match is the adjacent (2,3)<->(3,3)