Add overload FbVarChar::set function for non null-terminated string

[asfernandes]

Currently we have method void FbVarChar::set(const char* s) for null-terminated string.

It's good to have an overload void FbVarChar::set(const char* s, unsigned len) for non null-terminated strings.

[aafemt]

Methods accepting std::string and std::wstring also would be good.