cmake: add and test "unity" builds

"Unity" (aka "jumbo", aka "amalgamation" builds concatenate source files
before compiling. It has these benefits for example: faster builds,
improved code optimization, cleaner code. Let's support and test this.

- enable unity builds for some existing CI builds to test this build
  scenario.
- tune `UNITY_BUILD_BATCH_SIZE` size.
- disable unity build for example and test programs (they use one source
  each already).

You can enable it by passing `-DCMAKE_UNITY_BUILD=ON` to cmake.
Supported by CMake 3.16 and newer.

Ref: https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html

Closes #1034

example/CMakeLists.txt

@@ -48,6 +48,7 @@ foreach(example ${EXAMPLES})
   # to find generated header
   target_include_directories(${example} PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/../src ../src)
   target_link_libraries(${example} ${LIB_SELECTED} ${LIBRARIES})
+  set_target_properties(${example} PROPERTIES UNITY_BUILD false)
 endforeach()
 
 add_target_to_copy_dependencies(