Replace Program with TranslationUnit
Add VariableDefinitionData

Combine MutableVariableDefinitionData
and ImmutableVariableDefinitionData
into VariableDefinitionData.
Replace Program with TranslationUnit
Use VariableDefinitionData class

Use VariableDefinitionData
instead of MutableVariableDefinitionData
and ImmutableVariableDefinitionData.
Add and use parse_else_statement() member
Add and use parse_parameter() member
Add and use parse_function_call() member
Add and use parse_parenthesized_expression() member
Add and use parse_identifier_expression() member
Add and use parse_integer_literal_expression() member
Add CharacterLiteral and CharacterLiteralData
Add StringLiteral and StringLiteralData
Add parse_character_literal() member
Add parse_string_literal() member
Add CharacterLiteralExpressionData
Add StringLiteralExpressionData
Add parse_character_literal_expression() member
Add parse_string_literal_expression() member