Replace pretty printer by generation of AST, followed by a scan of the AST.

Replace pretty printer by generation of AST, followed by a scan of the AST.

This allows external users to perform their own pretty printing
or AST generation, without duplicating most of the code in pprint.c.
They simply need to first call

    root = cloog_clast_create(program->loop, equal, infos);

then scan the resulting struct clast_stmt and finally release the
clast_stmt:

    cloog_clast_free(root);

The clast tree structure is described in clast.h;
an example of scanning this structure can be found in pprint.c.