Shift the project from a compiler to an interpreter

There will be not enought time to produce a compiler with
all the planed features - and the fact that the language
is not imperative, but functional, does not help.

We intend to add an interactive mode as a "compensation".

README.md: acknowledge the new objective.
Update the documentation to reflect the shift

system_overview.md, lexer.h: reword "compiler" as "interpreter";
namgin_conventions.txt, makefile: specify that the refered
    compiler is gcc, not our own.
Remove all references to name mangling

Name mangling would be best used in a compiler, where we need some
technique to differentiate between all overloads of a function in
compiled code.

mangling.md: remove
system_overview.md: remove reference to mangling.md

ast.h: As we aim an interpreter instead of a compiler, it is better to
    have non-owning pointers to the operator definitions. This
    was done to the hierarchy of classes below TreeNodeBody.
    The only attribute of TreeNodeBody was a mangled name,
    that served as a reference to the chosen operator version.
    This attribute was replaced by a pointer to an operator,
    rendering TreeNodeBody empty.
    (Maybe we will remove this class in future revisions.)

tree_build.h, tree_build.cpp: remove the getMangledOperatorName function

symbol.h, symbol_table.h, operator.h: removed references to mangling