Ramp up compiler warnings

[0ax1]

As of now, the clap-wrapper target uses the default minimum of compiler warnings. In order to catch as many errors as possible at compile time, additional warning groups such as -Wall, -Wextra and -Wpedantic should be considered.

After resolving all warnings for the added groups, we could think about enabling warnings as errors: -Werror.

[baconpaul]

The next branch turns on wall on gcc and clang and clang has them mostly clear now

[0ax1]

landed for clang/gcc in: #109

[baconpaul]

we are now -Wall -Wextra -Wpedantic -Werror on gcc and clang. Thanks for raising this.

I suggest we open a separate issue for MSVC and close this one when we make 0.5.0