refactor(frontends/lean): use extension objects to store lean default frontend data in the environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/metavar): avoid using unique names for default metavariable prefix

The problem is that unique names depend on the order compilation units are initialized. The order of initialization is not specified by the C++ standard. Then, different compilers (or even the same compiler) may produce different initialization orders, and consequently the metavariable prefix is going to be different for different builds. This is not a bug, but it makes unit tests to fail since the output produced by different builds is different for the same input file.
Avoiding unique name feature in the default metavariable prefix avoids this problem.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/environment): add weak reference to environment objects

We need weak references to environment objects because the environment has a reference to the type_checker and the type_checker has a reference back to the environment. Before, we were breaking the cycle using an "environment const &". This was a dangerous hack because the environment smart pointer passed to the type_checker could be on the stack. The weak_ref is much safer.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>