lang: slice ranges take the same bound markers as for-header ranges

xs[1..=3] (end inclusive), xs[0<..<4] (both exclusive), xs[..=2]
(prefix form with markers, implicit 0 start), xs[2<..] (open end,
exclusive start), and xs[..] (whole collection) — lowered as lo+1 /
hi+1 on the existing subslice op. Strings slice through the same path.
An explicit end marker requires an end expression, matching the
for-header rule.

Regression: examples/0052-basic-slice-range-bounds.sx.

examples/0052-basic-slice-range-bounds.sx

@@ -0,0 +1,32 @@
+// Slice range bound markers — same matrix as for-header ranges: each side
+// of `..` takes `=` (inclusive) or `<` (exclusive), defaults 0-inclusive
+// start / exclusive end. Prefix form takes markers too ([..=2], [<..3]);
+// [..] is the whole slice; bounds are arbitrary expressions; strings slice
+// through the same path.
+
+#import "modules/std.sx";
+
+dump :: (s: []s64, tag: string) {
+    print("{}: ", tag);
+    for s (v) { print("{} ", v); }
+    print("(len {})\n", s.len);
+}
+
+main :: () -> s32 {
+    xs : [6]s64 = .[10, 11, 12, 13, 14, 15];
+    full : []s64 = xs[0..6];
+
+    dump(full[1..=3],  "1..=3");     // 11 12 13
+    dump(full[0<..<4], "0<..<4");    // 11 12 13
+    dump(full[..=2],   "..=2");      // 10 11 12
+    dump(full[<..3],   "<..3");      // 11 12
+    dump(full[2<..],   "2<..");      // 13 14 15
+    dump(full[..],     "..");        // all six
+    x := 3;
+    dump(full[x-1..=x+1], "x-1..=x+1"); // 12 13 14
+
+    s := "abcdef";
+    print("str 1..=3: {}\n", s[1..=3]);   // bcd
+    print("str 0<..<4: {}\n", s[0<..<4]); // bcd
+    0
+}

examples/expected/0052-basic-slice-range-bounds.exit

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

examples/expected/0052-basic-slice-range-bounds.stderr

@@ -0,0 +1 @@
+

examples/expected/0052-basic-slice-range-bounds.stdout

@@ -0,0 +1,9 @@
+1..=3: 11 12 13 (len 3)
+0<..<4: 11 12 13 (len 3)
+..=2: 10 11 12 (len 3)
+<..3: 11 12 (len 2)
+2<..: 13 14 15 (len 3)
+..: 10 11 12 13 14 15 (len 6)
+x-1..=x+1: 12 13 14 (len 3)
+str 1..=3: bcd
+str 0<..<4: bcd