Audit `iana-time-zone`

[pinkforest]

As described: https://github.com/strawlab/iana-time-zone/issues/64

This crate is now used by chrono and has become very important in the ecosystem

All these blocks seem documented

$ naive grep -R unsafe *

src/tz_android.rs:    let key = unsafe { CStr::from_bytes_with_nul_unchecked(b"persist.sys.timezone\0") };
src/tz_android.rs:        unsafe { PROPERTIES = Some(properties) };
src/tz_android.rs:    unsafe { PROPERTIES.as_ref() }
src/tz_windows.rs:    unsafe { Ok(get_timezone()?) }
src/tz_windows.rs:unsafe fn get_timezone() -> Result<String, HRESULT> {
src/tz_windows.rs:unsafe fn initialize_factory() {
src/tz_windows.rs:            unsafe { WindowsDeleteString(string) };
src/tz_windows.rs:            unsafe { (*(instance as *mut IUnknown)).Release() };
src/tz_macos.rs:    unsafe { get_timezone().ok_or(crate::GetTimezoneError::OsError) }
src/tz_macos.rs:unsafe fn get_timezone() -> Option<String> {
src/tz_macos.rs:        unsafe { CFRelease(self.0 as CFTypeRef) };
src/tz_macos.rs:    unsafe fn new(v: *const T) -> Option<Self> {

[lopopolo]

@pinkforest since you've expressed interest in this, just giving you and the safety dance team a heads up that we've significantly refactored the macOS internals if y'all are interested in taking a look. We're gonna let this PR sit until October 6 before merging it:

• https://github.com/strawlab/iana-time-zone/pull/67