Fix compiler crash related to using tuples as a generic constraint

[SeanTAllen]

Tuple types aren't legal constraints on generics in Pony and we have a check in
the syntax pass to detect and report that error. However, it was previously
possible to sneak a tuple type as a generic constraint past the syntax check by
smuggling it in a type alias.

Type aliases aren't flattened into their various components until the flatten
pass which follows the syntax pass. This means that the following code would
cause a compiler assertion:

type Blocksize is (U8, U32)
class Block[T: Blocksize]

This commit adds an additional check in the flatten pass to report an error on
smuggled tuple types as generic constraints.

Fixes #3655

[SeanTAllen]

Windows failure can be ignored.

[ergl]

I'd like to try if this catches something like the following:

type Blocksize is (None | (U8, U32))
class Block[T: Blocksize]

but I'm okay with merging now and fixing that edge case later

[SeanTAllen]

@ergl i just tested, it doesn't fix that case. let me see how hard that is to address as well.

[SeanTAllen]

@ergl can you open an issue for the type union one? its fairly easy to address but I'm too tired right now to deal with reworking the code to do the recursion necessary so I'm going to merge this now and then when the other issue is open, I or someone else can tackle.