gccrs: Add const checker

gccrs: Add const checker

Similarly to the unsafe checker, constant evaluation can only be performed
in a few contexts and include restrictions on the Rust language. Should
the user fail to uphold those conditions, errors will be reported and the
compilation pipeline interrupted.

These contexts are as follow:

- Array type length expressions
- Array repeat length expressions
- Constants
- Statics
- Enum discriminants
- Const generic arguments

In these contexts, the user is restricted to calling only functions marked
as `const` or perform arithmetic operations only on certain types, among
other restrictions.

	gcc/rust/
	* checks/errors/rust-const-checker.cc: New.
	* checks/errors/rust-const-checker.h: New.