Add file variable.h, wich contains the class Variable

This class will be used to pass data around by the program.
Since they are the dynamic part of the program, we optimized
them for size.
variable.h: Add constructors and destructors to Variable class.
variable.h: Disallow copies and moves of Variable

In theory, it is possible to implement correctly and efficiently
the code for copy, move and assignment of Variable, but since
these operations will not be used (except maybe for cloning),
this extra effort does not worth it.

Note: technically, copy and move constructors and assignment
operators are already "implicitly deleted because the default
definition would be ill-formed".
Explicitly deleting merely improve error messages.
variable.h: Add method Variable::clone

This should resolve all possible problems with
the disallowance of copies.
variable.h: Add operator== for Variable.
variable.h: Add class VariableTable.
variable.h: Move methods implementation to variable.cpp.
variable.h: Improve documentation.
Rename LocalSymbolTable to VariableList

Affected files: symbol_table.cpp, symbol_table.h, tree_build.cpp.
symbol_table.h: Correct documentation at beginning of file

We forgot to mention the change mate in revision dc912f02
("Substituted the global symbol table class for a namespace")
in the documentation present at the beginning of the file.
Rename GlobalSymbolTable to SymbolTable

Since the old LocalSymbolTable is now VariableList, the
GlobalSymbolTable can take its place as "the" SymbolTable.

Affected files: semantyc_analyser.cpp, semantic_analyser.h,
symbol_table.cpp, symbol_table.h, tree_build.cpp.
ast.cpp: Intercalate each class with a commentary line.
ast.h: Add method OperatorParameter::decompose and its documentation.
ast.cpp: Implement OperatorParameter::overload.
ast.h: Add method OperatorBody::evaluate and its documentation.
operator.h: Add evaluation methods to operators

They were named 'compute' instead of 'evaluate' to distinguish
them from the OperatorBody::evaluate methods.
ast.cpp: Implement 'evaluate' methods.
operator.cpp: Implement Overload::compute methods.
operator.h: Implement Operator::compute methods.
variable.cpp: Implement VariableTable.