Add checks for constant evaluation

[CohenArthur]

Handle all const contexts properly:

• Array type length expressions
• Array repeat length expressions
• constants
• statics
• enum discriminants
• const generic arguments

Handle allowed subset properly in const contexts:

• Literals.
• Const parameters.
• Paths to functions and constants. Recursively defining constants is not allowed.
• Paths to statics. These are only allowed within the initializer of a static.
• Tuple expressions.
• Array expressions.
• Struct expressions.
• Block expressions, including unsafe blocks.
• let statements and thus irrefutable patterns, including mutable bindings
• assignment expressions
• compound assignment expressions
• expression statements
• Field expressions.
• Index expressions, array indexing or slice with a usize.
• Range expressions.
• Closure expressions which don't capture variables from the environment.
• Built-in negation, arithmetic, logical, comparison or lazy boolean operators used on integer and floating point types, bool, and char.
• Shared borrows, except if applied to a type with interior mutability.
• The dereference operator except for raw pointers.
• Grouped expressions.
• Cast expressions, except
• pointer to address casts,
• function pointer to address casts, and
• unsizing casts to trait objects.
• Calls of const functions and const methods.
• loop, while and while let expressions.
• if, if let and match expressions.
• Mutable references are forbidden in const functions #1552