Minor code cleanup.

Removed unused usings, renamed UnarySyntax and BinarySyntax, removed FunctionDeclarationSyntax (integrated into FunctionSyntax) and removed unused code.
Refactored VariableDeclarationSyntax and made CommaOperatorSyntax expressions ExpressionSyntax nodes.
Made the syntax tree and parsers internal.
Moved target variables to the constructors.
Removed IForStatement.
Corrected ForEachIn.

ForEachIn now correctly specifies a variable identifier and target.
Introduced BodySyntax.

Introduced BodySyntax to manage the body of a function or program.
Simplified and fixed SwitchSyntax.

The DefaultClause has been removed and merged into CaseClause (renamed to SwitchCase). Also, the generation of the switch structure was incorrect. Added test cases and fixed bugs.
Renamed VisitUnaryExpression and VisitBinaryExpression.
Removed SyntaxExpressionType.Power.

Don't know how it ended up there.
Factored out genericArguments.