Name shadowing

[sylefeb]

Variable name shadowing currently produces an error (draft), however a strong warning may be better as there are situations where this acceptable (see e.g. #108 (comment)).

Overall name shadowing needs a complete pass to clarify and revise cases of name collisions (unit/subroutine/algorithm/variable/circuitry/instances).

Only clear error cases should trigger an error, e.g.:

int a = 0;
int a = 5; // this cannot be good

In such cases, the internal variable name truly collides anyway...