GeezerVison

Fixes #3057.

Changes proposed in this pull request:
An explicit cast of a small integer (i8, u8, i16, u16) to bool (<bool>x, x as bool, bool(x)) now clears the bits above the small type before testing the value. Before, convertExpression only wrapped small integers when converting to a larger type, and bool (size 1) never qualified, so a value that overflowed to 0 in its own type still tested as true:

function f(a: u8): bool {
  let c: u8 = a + 1;
  return <bool>c;
}
f(255); // true before, false now (c is 0)

The conditional form (c ? true : false) already went through makeIsTrueish, which wraps, so only the explicit cast was affected.

Added cases to tests/compiler/bool.ts for u8, i8, u16 and i16 values that wrap to 0 (and some that don't). With the old compiler the module aborts on the first new assertion; with the fix it runs. Regenerated the bool and cast fixtures; cast.debug.wat changes because it contains such casts (an i32.and/sign-extension now precedes the i32.ne). npm run test:compiler passes.

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