sdk: introduce Spice SDK (#676)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds a flat-C, headers-only library for writing DLL plugins with spice.

This is meant to be an alternative to writing applications over spice
API which doesn't suit some scenarios like writing custom
high-performance I/O modules.

All you have to do is:

1. Create a 32-bit or 64-bit DLL with `spice_sdk_entry_point` as a
DLLExport
1. Include `spicesdk.h`, and optionally `spicesdk_io.h` (see
`extras\sdk\include`)
1. `spice_sdk_entry_point` will be called when hooks get initialized.
Call `init` with valid parameters.
1. When `init` returns, `SPICE_SDK_V0` will be filled out with a series
of function pointers; your DLL can now start calling into it.
1. Compile your DLL and add it as a DLL hook (`-k`) in spice when
launching the game.

See `v0_cpp.cpp` for an example.

V0 implements the following functions:

```
    log;

    get_game_info;
    get_avs_info;

    get_button;
    set_button;

    get_analog;
    set_analog;

    get_light;
    set_light;

    set_touch;
    clear_touch;

    insert_card;
    set_keypad;
```

V0 is focused on providing common utility functions needed in a lot of
DLL hooks (get AVS info, log messages to file), and implementing I/O.

In the future, we could have more interesting things like:

* scanning and modifying memory 
* installing DLL hooks
* having direct hooks into I/O emulation modules
* getting full tape LEDs
* having a callback into DX9 `Present` call
* drawing an ImGui window

More detailed documentation is coming soon in form of a wiki page.

## Testing
See included sample DLL.

src/spice2x/README.md

@@ -283,6 +283,16 @@ Python is the only one that is fully spec compliant.
 Other libraries may be missing features and contain bugs; please feel free to
 contribute code to fill the gaps if you work on a project using these libraries.
 
+## Spice SDK (for DLL hooks / plugins)
+
+As an alternative to Spice API, Spice SDK is available.
+
+This is a flat C, header-only library that can be included in your DLL. Once
+initialized, you can call directly into helper routines provided by Spicetools
+executable, making low-latency and high-throughput interactions possible.
+
+Check the documentation on the wiki for more details.
+
 ## License
 Unless otherwise noted, all files are licensed under the GPLv3.
 See the LICENSE file for the full license text.