## Summary of the Pull Request
Introduces convenience type `til::rectangle` which automatically implements our best practices for rectangle-related types and provides automatic conversions in/out of the relevant types.
## PR Checklist
* [x] In support of Differential Rendering #778
* [X] I work here.
* [x] Tests added/passed
* [x] I'm a core contributor.
## Detailed Description of the Pull Request / Additional comments
- Automatically converts in from anything with a Left/Top/Right/Bottom or left/top/right/bottom (Win32 `RECT`)
- Automatically converts Console type `SMALL_RECT` and shifts it from **inclusive** to **exclusive** on instantiation
- Automatically converts out to `SMALL_RECT` (converting back to **inclusive**), `RECT`, or `D2D1_RECT_F`.
- Constructs from bare integers written into source file
- Constructs from a single `til::point` as a 1x1 size rectangle with top-left corner (origin) at that point
- Constructs from a single `til::size` as a WxH size rectangle with top-left corner (origin) at 0,0
- Constructs from a `til::point` and a `til::size` representing the top-left corner and the width by height.
- Constructs from a `til::point` and another `til::point` representing the top-left corner and the **exclusive** bottom-right corner.
- Default constructs to empty
- Uses Chromium numerics for all basic math operations (+, -, *, /)
- Provides equality tests
- Provides `operator bool` to know when it's valid (has an area > 0) and `empty()` to know the contrary
- Accessors for left/top/right/bottom
- Type converting accessors (that use safe conversions and throw) for left/top/right/bottom
- Convenience methods for finding width/height (with Chromium numerics operations) and type-converting templates (with Chromium numerics conversions).
- Accessors for origin (top-left point) and the size/dimensions (as a `til::size`).
- Intersect operation on `operator &` to find where two `til::rectangle`s overlap, returned as a `til::rectangle`.
- Union operation on `operator |` to find the total area covered by two `til::rectangles`, returned as a `til::rectangle`.
- Subtract operation on `operator -` to find the area remaining after one `til::rectangle` is removed from another, returned as a `til::some<til::rectangle, 4>`.
- TAEF/WEX Output and Comparators so they will print very nicely with `VERIFY` and `Log` macros in our testing suite.
- Additional comparators, TAEF/WEX output, and tests written on `til::some` to support the Subtract operation.
- A natvis
## Validation Steps Performed
- See automated tests of functionality.
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