register-type for generics

What if we allow something like:

```
(register-type (FixArray Int t))
```

That would then emit something like this for `(FixArray 256 Char)`, in the vein of https://github.com/carp-lang/Carp/pull/581:
```
#define INT0 256
#define T Char
#include "fixarray.h"
#undef T
#undef INT0
```
AFAICS it would make extending the language with new types easier and also would allow removing things like `Array` from the compiler.

Start by reviewing the register-type proposal and the implementation discussed in PR #581, then inspect how Array is currently handled by the compiler. Determine the design needed for generic type registration and preprocessing substitutions. Done means the example supports FixArray with generic parameters and the intended built-in Array handling can be removed.

Written by the indexing model from the issue text.