Use ReadOnlySpan<HwndWrapperHook> instead of a temporary heap array

[h3xds1nz]

Description

Avoids creation of temporary heap array when initiating HwndWrapper, contributing to startup performance.

• I've also replaced "" with string.Empty to use singletons in the API calls.
• I've also modified the hwnd class-name creation to avoid some allocations, logic of course stays the same. (to be honest, the best would be to cache the assembly name in case you're creating multiple windows, since now this needs to be retrieved everytime though Minor Performance Optimization by Reducing Calls to AppDomain.CurrentDomain.FriendlyName #8567 has optimized this to at least be just 1 call per invocation but could be zero after 1st retrieval, really).
• Tons of unnecessary usings from touched files were removed.
• Also removes HWND struct as a bonus (one of many, this one is unsed and also not particularly perfect).

Customer Impact

Increased performance, decreased allocations.

Regression

No.

Testing

Local build, app run.

Risk

Low.

Microsoft Reviewers: Open in CodeFlow