Add SimpleCommandWordParserBase class
Add WordComponent::appendConstantValue method
Add Word::constantValue method

To ensure consistency of the cached constant value, the Word class's
data members are now effectively constant.
Add WordComponent::isRawString method
Add Word::isSingleRawString method
Add Keyword class
Add Maybe::pointer method
Parse keyword in SimpleCommandWordParserBase
Add SimpleCommandWordParser
Rename SimpleCommandParser

The SimpleCommandParser class is going to be replaced by a new
implementation. The old implementation is temporarily kept in a
different name.