[Refactoring] Clean up syntaxCopy functions

Dsymbol, Statement, Expression, Type, and Initializer.
[Refactoring] Parser::parseCtor()
[Refactoring] modToChars and modToBuffer
[Refactoring] stcToBuffer and stcPickToBuffer
[Refactoring] Print tthis in argTYpesToBuffer
[Refactoring] Reduce indent level
[Refactoring] Fix signature of the constructor in FuncExp
[Refactoring] Add Parser::parseFunctionLiteral()
[Refactoring] IndexExp::semantic
[Refactoring] SliceExp::semantic
[Refactoring] fix indent in declaration.c and optimize.c
FuncExp::semantic