unwind_protect and safe[] can only be used for SEXP or void return

[bkietz]

This introduces unnecessary boilerplate when using a function like Rf_translateCharUTF8 which returns const char*:

cpp11/inst/include/cpp11/as.hpp

Lines 134 to 136 in 030f91d

	const char* c_p = nullptr;
	unwind_protect([&] { c_p = Rf_translateCharUTF8(STRING_ELT(from, 0)); });
	return c_p[0];

It would be fairly straightforward to extend these to allow:

      return unwind_protect([&] { return Rf_translateCharUTF8(STRING_ELT(from, 0)); })[0];

Or even (provided STRING_ELT doesn't require protection):

      return safe[Rf_translateCharUTF8](STRING_ELT(from, 0))[0];

[jimhester]

Agreed, however most R API functions return a SEXP or are void, so if supporting this would produce a bunch of complexity we would probably be ok to leave it out. If it is not too bad then sure it would be nice to handle any type of return value.