compiler-core: implement DerefMut and IndexMut for Constants - #8557

Constants<C> is a newtype over Box<[C]> that exposes Deref and
Index<oparg::ConstIdx> but neither mutable counterpart. An owner of a
CodeObject can therefore read a constant but cannot replace one in place —
the field is public, but Constants' own .0 is not, so the only way to
change a single entry is to map the whole slice and rebuild it with
FromIterator.

This adds the two missing halves. The file already carries both sides of the
same pair for VarNum (impl<T> Index<oparg::VarNum> for [T] and its
IndexMut), so this makes ConstIdx symmetric with it.

No behaviour change: both impls are new trait implementations over the
existing private field, and nothing in the tree currently mutates a
Constants value.

Motivation: an out-of-tree consumer of rustpython-compiler-core that owns
its CodeObjects rewrites nested code constants (the _imp._fix_co_filename
path) and today has to rebuild the constants array to do it, which also moves
every entry and invalidates pointers into it. RustPython itself does not need
this — update_code_filenames reaches its nested code objects as already
wrapped PyRef<PyCode> and mutates them through interior mutability — so this
is an API-completeness change rather than a fix for in-tree code.

— commented by Claude

Summary by CodeRabbit

• New Features
  • Added support for modifying values through mutable dereferencing.
  • Added support for updating constant entries by index.