Upgrade to C++17

[lorenzo-gomez-windhover]

std::optional, which is added in C++17, would allow to return "empty" values from functions in a concrete/explicit way and prevent us from using "sentinel" polymorphic variables, improving readability in the intent of code.

For example:

bool      Symbol::hasEncoding() { return encoding != -1; }

Can be converted to something like

std::optional<int>    Symbol::getEncoding() { encoding; }

and use a function like value_or to concretely express the intent without having the ambiguity of using a "-1" for "invalid"

Changes between C++14 and C++17

Compiler support for C++17