Deduce return type for all API invokable functions.

[accelerated]

Signed-off-by: Alexander Damian [email protected]

Describe your changes

• Automatically deduce the return type for all callable objects passed to the library. This means that it is not longer required/needed to specify the return type as a template parameter when when posting jobs, making it easier and more intuitive. This applies to V1 and V1 APIs including forEach and mapReduce functions.
  As an example:

 auto ctx = dispatcher.postFirst<int>([](CoroContext<int>::Ptr ctx)->int {
        return ctx->set(55); //declared type is int
    })->then<double>([](CoroContext<double>::Ptr ctx)->int {
        return ctx->set(22.33); //declared type is double
    })->then<std::string>([](CoroContext<std::string>::Ptr ctx)->int {
        return ctx->set("future"); //declared type is string
    })->then<std::list<int>>([](CoroContext<std::list<int>>::Ptr ctx)->int {
        return ctx->set(std::list<int>{1,2,3}); //declared type is list<int>
    })->end();

now becomes

auto ctx = dispatcher.postFirst([](CoroContext<int>::Ptr ctx)->int {
        return ctx->set(55); //deduced type is int
    })->then([](CoroContext<double>::Ptr ctx)->int {
        return ctx->set(22.33); //deduced type is double
    })->then([](CoroContext<std::string>::Ptr ctx)->int {
        return ctx->set("future"); //deduced type is string
    })->then([](CoroContext<std::list<int>>::Ptr ctx)->int {
        return ctx->set(std::list<int>{1,2,3}); //deduced type is list<int>
    })->end();

Notice that post(), then()... do not take template arguments anymore. Similarly for forEach or mapReduce:

    std::vector<int> start{0,1,2,3,4,5,6,7,8,9};
    std::vector<char> results = getDispatcher().forEach<char>(start.cbegin(), start.size(),
        [](VoidContextPtr, const int& val)->char {
        return 'a'+val;
    })->get();

becomes

    std::vector<int> start{0,1,2,3,4,5,6,7,8,9};
    std::vector<char> results = getDispatcher().forEach(start.cbegin(), start.size(),
        [](VoidContextPtr, const int& val)->char {
        return 'a'+val;
    })->get();

Testing performed
Compiled and ran unit tests

Additional context

1. Note that this change is backwards compatible and no code changes are needed. Going forward, new code can simply omit the template parameters
2. In the next major release, the template parameters marked with Deprecated may be removed to enforce simplicity, thus braking API.