Merge #1527 #1529 #1530 #1534

1527: rust: Add -frust-compile-until option r=CohenArthur a=CohenArthur

This option helps ensure that we do not introduce regressions on various
parts of the compilation pipeline. For example, a testcase (or testsuite
from the `testing` project) might pass attribute checking, expansion and
lowering, but fail during typechecking. Should a change suddenly make
that testcase fail expansion, we would not be able to notice it. By
generating tests that run up until expansion, typechecking, compilation
and so forth we ensure that no regressions are added accidentally to
already failing tests/testsuites.


1529: const generics: Make sure const generic types are visited properly in all contexts r=CohenArthur a=CohenArthur

Closes #1360 

1530: const generics: Forbid default values in Functions, Traits and Impls r=CohenArthur a=CohenArthur

Fixes #1310 

1534: module lowering: Do not append null pointers as items r=CohenArthur a=CohenArthur

Some module items do not need to get lowered to HIR such as `macro_rules!` definitions. Hence, module lowering should act the same as crate lowering: Only emplace back the lowered item if it is a valid pointer

Fixes #1533 

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>