replace is_callable_v workaround with is_invocable_v in eos-vm; fixes clang12 build

[spoonincode]

Change Description

This is_callable_v workaround in eos-vm for some old compiler is causing compile failures on clang12 with the error

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

Replacing the workaround with simply is_invocable_v seems to work on clang12, our entire CICD collection, and gcc8, clang6, and XCode 10.2 (the latter of which doesn't actually build the entirety of eosio develop anyways). Please review carefully though I may be misunderstanding what is going on here.

Needs EOSIO/eos-vm#210 and seems to pick up EOSIO/eos-vm#209 which hasn't been synced up here.

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