Disallow multiple definitions of the same name

[kach]

This kind of thing should not be allowed in the compiler (but ideally should be allowed in the REPL for the sake of experimentation). The check can be implemented as part of the typechecker.

define constant x = 5
define constant x = 6

[jeromesimeon]

I don't see a problem with this code. Also having the REPL behavior differ from the one in Ergo is a nuisance since we have to maintain both behaviors.

[jeromesimeon]

Enums support is based on constants, which means there can be shadowing. We should make this a compiler warning (using the recently added #300 support).

[jeromesimeon]

This has been made a warning (#699 )