string_view version of `script::find_symbol_by_name` and few others

[Try]

Current syntax: symbol* script::find_symbol_by_name(const std::string& name) forces client to bake std::string into interface. for find function this is especially unnecessary, since it does string copy immediatly after:

    symbol* script::find_symbol_by_name(const std::string& name) {
        std::string up {name}; // <-- fine, but no need to query name as string
        std::transform(up.begin(), up.end(), up.begin(), ::toupper);

        if (auto it = _m_symbols_by_name.find(up); it != _m_symbols_by_name.end()) {
            return find_symbol_by_index(it->second);
        }

        return nullptr;
    }

In general it's probably would be good to pass read-only strings as string_view in all cases. If implementation then uses this string as key to has-map or similar - solve this locally inside the function.

Other places:

R vm::call_function(const std::string& name, P... args)
vm::init_instance(const std::string& name)
vm::allocate_instance(const std::string& name)
void vm::push_string(const std::string& value)
void register_external(const std::string& name, const std::function<R(P...)>& callback)
void override_function(const std::string& name, const std::function<R(P...)>& callback)
const way_point* way_net::waypoint(const std::string& name) const
const message_block* messages::block_by_name(const std::string& name) const

[lmichaelis]

Partial duplicate of #21, but a good point. Until I can figure out a C++17 compatible way of using string-views all the way down, this is probably a good solution; especially for case-insensitive strings.

[lmichaelis]

Since I've been able to implement most of these as std::string_view I'm closing this issue. I have deprecated waynet::waypoint since that API is broken anyways.