replace is_callable_v workaround with is_invocable_v; fixes build on clang12

[spoonincode]

This removes the is_callable_v workaround and replaces it with std::is_invocable_v as the workaround is causing a

template argument for non-type template parameter must be an expression

error on clang12. Fixes clang12 and also is compiling on gcc8, clang6, and XCode 10.2 -- pretty much the bare minimum for eosio. But, I'm not sure if it's desirable for eos-vm to support a wider range of compilers and the workaround should be fixed instead. Or if I've just completely missed the purpose of this code in the first place... so please review carefully.

[swatanabe]

is_invokable_v is wrong. is_callable_v is used here to distinguish (member) function pointers from function objects. OTOH, I have no idea how the is_callable_v ever worked, as clang is obviously correct to diagnose it as an error.

[spoonincode]

It would be useful to add a unit test that exercises this code then, as far as I can remember all unit tests did pass with is_invocable_v