Analyser: allow enum type as array length in array definitions

[chqrlie]

• arrays defined with an enum type as the length must be indexed using enum values of this type
• enum values are scoped in array subscript expressions and initializers

Implements #403

[bvdberg]

Maybe we could change the diagnostic 'use of undeclared identifier' to 'array subscript must have enum type 'test.Enum'' also? It's weird that a[No] works and a[No +1] says that No is unknown..

[chqrlie]

I agree the diagnostic is confusing but this problem is not new: Enum e = No + 1; has the same problem.

To solve this, we would need to add the enum at the top of the scope stack so the expression can be resolved. Yet doing so it not completely correct as No would then resolve even in complex expressions where it is possibly ambiguous and inappropriate: Enum e = some_function_that_takes_another_enum(No);

[bvdberg]

I think the number of tests are a bit limited. I can think of many more:

u32[Enum] b = { 10, 20, 30 }

static_assert(3, sizeof(char[Enum]));

in function:
b[Enum.Yes] = 10;

inside a struct:
type Struct struct {
char[Enum] cols;
}

as type:
type Foo char[Enum];
public type Bar char[Enum]; // @error{public declaration using non-public variable 'test.Color'}

In function scope:
Struct s;
s.cols[(Enum)10] = 20;
-> error: array out-of-bounds access [10] in array of [3]
seems a good error, but good to know that that keeps working in unit test

multi-dimensional, both one/both dimensions the Enum.
Pointer to Enum: u32[Enum*] ..