require parentheses with shifting and arithmetic

[andrewrk]

This is a sub-task of #114.

const std = @import("std");

pub const max_ciphertext_len = 1 << 14 + 256;

test "operator precedence" {
    try std.testing.expect(max_ciphertext_len == 16640);
}

Actual behavior:

$ stage3/bin/zig test test.zig 
Test [1/1] test.operator precedence... FAIL (TestUnexpectedResult)
/home/andy/dev/zig/lib/std/testing.zig:509:14: 0x20bed7 in expect (test)
    if (!ok) return error.TestUnexpectedResult;
             ^
/home/andy/dev/zig/build-release/test.zig:6:5: 0x20bff5 in test.operator precedence (test)
    try std.testing.expect(max_ciphertext_len == 16640);
    ^
0 passed; 0 skipped; 1 failed.

Expected behavior:

test.zig:3:39: error: parentheses required to disambiguate operator precedence
pub const max_ciphertext_len: u16 = 1 << 14 + 256;
                                    ~~^~~~~~~~~~~