Moved ostream includes do .cpp and kept only iosfwd

Translation units like build_tree need not to know
about the printing operators of ostream.

printable.h, token.h: change #include<ostream> to #include<iosfwd>
ast.cpp, operator.cpp: add #include<ostream>.
ast.cpp: Implement missing classes below TreeNodeBody.
Implement missing classes VariableBody and NumericBody

ast.cpp: Implementation of the mehods of these calsses
ast.h: Rename constructor parameters for consistency.
symbol_table.cpp: Implement LocalSymblolTable::merge.
symblol_table.h Correct GlobalSymbolTable::insertOverload signature

This final change makes the code lo link.
But it still needs testing and debugging.