Use < > brackets for effects

With the introduction of using `{}` in type signatures to implicitly quantify over variables, there are now 3 distinct uses of curly braces that all show up in type signatures:

1. implicit quantification
2. records
3. effects

The effect syntax also uses vertical bars `|`, which make their syntax confusingly close to that of variants. For example, here are two different type signatures in the prelude:

One to do with effects:
```
def liftState {a b c h eff} (ref: Ref h c) (f:a -> {|eff} b) (x:a) : {State h|eff} b =
```

One to do with variants:
```
Iso {foo: a | ...b} (a | {|...b}))
```

So I suggest we use angle brackets `< >` for effects. They aren't used at all in the current syntax. They would also give a more explicit nod to Koka for developing some of these ideas.

No files, tests, or entry points are named. Begin by locating the parser and type-signature handling for effects and variants, then review how syntax changes are tested; done means effect signatures use angle brackets without ambiguity with records, quantification, or variants.

Written by the indexing model from the issue text.