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/CMakeLists.txt

@@ -607,6 +607,9 @@ set(SOURCE_FILES ${SOURCE_FILES}
         reader/structuredmessage.cpp
         reader/crypt.cpp
 
+        # sdk
+        sdk/sdk.cpp
+
         # stubs
         stubs/stubs.cpp
 
@@ -860,6 +863,36 @@ if(NOT MSVC)
     set_target_properties(spicetools_stubs_cpusbxpkm PROPERTIES COMPILE_FLAGS "-m32" LINK_FLAGS "-m32")
 endif()
 
+# sdk_sample_v0_flat_c.dll (32 bit)
+set(SOURCE_FILES sdk/sample/v0/flat_c/v0_flat_c.c)
+add_library(spicetools_sdk_sample_v0_flat_c_32 SHARED ${SOURCE_FILES} ${RESOURCE_FILES} sdk/sample/v0/flat_c/v0_flat_c.def)
+set_target_properties(spicetools_sdk_sample_v0_flat_c_32 PROPERTIES PREFIX "")
+set_target_properties(spicetools_sdk_sample_v0_flat_c_32 PROPERTIES OUTPUT_NAME "sdk_sample_v0_flat_c")
+
+if(NOT MSVC)
+    set_target_properties(spicetools_sdk_sample_v0_flat_c_32 PROPERTIES COMPILE_FLAGS "-m32" LINK_FLAGS "-m32")
+endif()
+
+# sdk_sample_v0_flat_c.dll (64 bit)
+set(SOURCE_FILES sdk/sample/v0/flat_c/v0_flat_c.c)
+add_library(spicetools_sdk_sample_v0_flat_c_64 SHARED ${SOURCE_FILES} ${RESOURCE_FILES} sdk/sample/v0/flat_c/v0_flat_c.def)
+set_target_properties(spicetools_sdk_sample_v0_flat_c_64 PROPERTIES PREFIX "")
+set_target_properties(spicetools_sdk_sample_v0_flat_c_64 PROPERTIES OUTPUT_NAME "sdk_sample_v0_flat_c")
+
+if(NOT MSVC)
+    set_target_properties(spicetools_sdk_sample_v0_flat_c_64 PROPERTIES COMPILE_FLAGS "-m64" LINK_FLAGS "-m64")
+endif()
+
+# sdk_sample_v0_cpp.dll (64 bit)
+set(SOURCE_FILES sdk/sample/v0/cpp/v0_cpp.cpp)
+add_library(spicetools_sdk_sample_v0_cpp_64 SHARED ${SOURCE_FILES} ${RESOURCE_FILES} sdk/sample/v0/cpp/v0_cpp.def)
+set_target_properties(spicetools_sdk_sample_v0_cpp_64 PROPERTIES PREFIX "")
+set_target_properties(spicetools_sdk_sample_v0_cpp_64 PROPERTIES OUTPUT_NAME "sdk_sample_v0_cpp")
+
+if(NOT MSVC)
+    set_target_properties(spicetools_sdk_sample_v0_cpp_64 PROPERTIES COMPILE_FLAGS "-m64" LINK_FLAGS "-m64")
+endif()
+
 # output directories
 ####################
 
@@ -869,14 +902,14 @@ set_target_properties(spicetools_cfg spicetools_cfg_linux
         RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/spicetools")
 
 # output 32bit
-set_target_properties(spicetools_spice spicetools_spice_laa spicetools_spice_linux spicetools_stubs_kbt spicetools_stubs_kld spicetools_stubs_cpusbxpkm
+set_target_properties(spicetools_spice spicetools_spice_laa spicetools_spice_linux spicetools_stubs_kbt spicetools_stubs_kld spicetools_stubs_cpusbxpkm spicetools_sdk_sample_v0_flat_c_32
         PROPERTIES
         ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/archive32"
         LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/spicetools/32"
         RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/spicetools/32")
 
 # output 64bit
-set_target_properties(spicetools_spice64 spicetools_spice64_linux spicetools_stubs_kbt64 spicetools_stubs_kld64 spicetools_stubs_nvcuda spicetools_stubs_nvcuvid spicetools_stubs_nvEncodeAPI64
+set_target_properties(spicetools_spice64 spicetools_spice64_linux spicetools_stubs_kbt64 spicetools_stubs_kld64 spicetools_stubs_nvcuda spicetools_stubs_nvcuvid spicetools_stubs_nvEncodeAPI64 spicetools_sdk_sample_v0_flat_c_64 spicetools_sdk_sample_v0_cpp_64
         PROPERTIES
         ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/archive64"
         LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/spicetools/64"