first_of_month @intCast

[supxinfy]

For some reason an attempt to build this library for Windows ends up with an error related to type mismatch like below. That is not the case for Linux or Mac. This PR adds @intCast to const first_of_month definitions to solve this issue.

C:\Users\runneradmin\AppData\Local\zig\p\zeit-0.6.0-5I6bk36tAgATpSl9wjFmRPMqYN2Mn0JQHgIcRNcqDpJA\src\timezone.zig:864:81: error: expected type 'i32', found 'i64'
        const first_of_month = zeit.weekdayFromDays(days_from_epoch - time.wDay + 1);
                                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
C:\Users\runneradmin\AppData\Local\zig\p\zeit-0.6.0-5I6bk36tAgATpSl9wjFmRPMqYN2Mn0JQHgIcRNcqDpJA\src\timezone.zig:864:81: note: signed 32-bit int cannot represent all possible signed 64-bit values
C:\Users\runneradmin\AppData\Local\zig\p\zeit-0.6.0-5I6bk36tAgATpSl9wjFmRPMqYN2Mn0JQHgIcRNcqDpJA\src\zeit.zig:1783:30: note: parameter type declared here
pub fn weekdayFromDays(days: Days) Weekday {
                             ^~~~