[C++ runtime] Improvements of Vocabulary::fromTokenNames

[IohannRabeson]

This commit doesn't change the behavior. It contains few small improvements to the class Vocabulary:

• use a reference for the variable tokenName to prevent a useless copy
• do not to check twice if tokenName is empty
• removed useless variable firstChar and use std::string::front() when possible
• use std::string::empty() instead of creating an empty string to compare with
• use std::string::clear() instead of assigning an empty string to clear a string
• use defaulted functions when possible

[mike-lischke]

@IohannRabeson Please pull the branch again and fix the conflict.

@parrt After the conflict has been solve this patch can be merged.