Runtime: systematically use array.get_u rather array.get for strings

This does not make any difference with binaryen which conflated both,
but the spec mandates the former.

Author: vouillon

runtime/wasm/bigstring.wat

@@ -158,7 +158,7 @@
                   (call $ta_get_ui8 (local.get $d1)
                      (i32.add (local.get $pos1) (local.get $i))))
                (local.set $c2
-                  (array.get $string (local.get $s2)
+                  (array.get_u $string (local.get $s2)
                      (i32.add (local.get $pos2) (local.get $i))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $loop (i32.eq (local.get $c1) (local.get $c2)))

runtime/wasm/float.wat

@@ -509,7 +509,7 @@
          (if (i32.lt_u (local.get $i) (local.get $len))
             (then
                (if (i32.eq (i32.const 32) ;; ' '
-                           (array.get $string (local.get $s) (local.get $i)))
+                           (array.get_u $string (local.get $s) (local.get $i)))
                   (then
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (br $skip_spaces))))))

runtime/wasm/parsing.wat

@@ -92,7 +92,7 @@
       (local $i i32)
       (local.set $i (local.get $p))
       (loop $loop
-         (if (i32.ne (array.get $string (local.get $s) (local.get $i))
+         (if (i32.ne (array.get_u $string (local.get $s) (local.get $i))
                (i32.const 0))
             (then
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -106,7 +106,7 @@
       (local $name (ref $string))
       (local.set $names (ref.cast (ref $string) (local.get $vnames)))
       (loop $loop
-         (if (i32.eqz (array.get $string (local.get $names) (local.get $i)))
+         (if (i32.eqz (array.get_u $string (local.get $names) (local.get $i)))
             (then
                (return
                   (array.new_data $string $unknown_token