Add comparison operators for Keyword and Operator
Add static methods to create Keyword and Operator
Fix createTrie making dangling references

The previous implementation of Operator::createTrie was wrongly using a
initializer list of strings, in which temporary copies of the static
member constant strings are created and references to them are stored in
the operator objects in the trie.

The new implementation uses a initializer list of operator objects which
already contains valid references to the static members.
Add SpecificOperatorParser class