Add `HSTRING` builder and registry support

[kennykerr]

This update adds two features to improve string support and efficiency.

The HSTRING type now includes an HStringBuilder type for pre-allocating and constructing an HSTRING without requiring an additional copy. This is similar to the WindowsPreallocateStringBuffer function but implemented directly in Rust, as I did for C++/WinRT back in the day.

const HELLO: [u16; 5] = [0x48, 0x65, 0x6C, 0x6C, 0x6F];
let mut b = HStringBuilder::new(5)?;
b.copy_from_slice(&HELLO);
let h: HSTRING = b.into();
assert_eq!(&h, "Hello");

The windows-registry crate now also provide get_hstring and set_hstring for reading and writing registry values using HSTRING. This is far more efficient than either String or OsString and is lossless, while still providing convertibility with String and OsString.

key.set_hstring("key", h!("value"))?;
let value: HSTRING = key.get_hstring("key")?;
key.set_hstring("key", &value)?;

Fixes: #3119

[kennykerr]

Thanks to @riverar for helping me fix that natvis... again. 😂