Without that, we may encounter weird error messages when writing
validators without an explicit `else`. Since we automatically fill it
with a `fail`; without annotation, it unifies to a generic parameter.
The existing check that would look for the body being an error term is
ill-advised as it doesn't work as soon as one adds tracing, or make
the validator a parameterized validator. Plus, it may simply trigger
the wrong behavior as one can now annotate a validator with _whatever_
and get pass the type-checker by plucking a `fail` keyword as body.
- We now consistently desugar an expect in the last position as
`Void`. Regardless of the pattern. Desugaring to a boolean value is
deemed too confusing.
- This commit also removes the desugaring for let-binding. It's only
ever allowed for _expect_ which then behaves like a side effect.
- We also now allow tests to return either `Bool` or `Void`. A test
that returns `Void` is treated the same as a test returning `True`.
This is debatable, but I would argue that it's been sufficiently
annoying for people and such a low-hanging fruit that we ought to do
something about it.
The strategy here is simple: when we find a sequence of expression
that ends with an assignment (let or expect), we can simply desugar it
into two expressions: the assignment followed by either `Void` or a
boolean.
The latter is used when the assignment pattern is itself a boolean;
the next boolean becomes the expected value. The former, `Void`, is
used for everything else. So said differently, any assignment
implicitly _returns Void_, except for boolean which return the actual
patterned bool.
<table>
<thead><tr><th>expression</th><th>desugar into</th></tr></thead>
<tbody>
<tr>
<td>
```aiken
fn expect_bool(data: Data) -> Void {
expect _: Bool = data
}
```
</td>
<td>
```aiken
fn expect_bool(data: Data) -> Void {
expect _: Bool = data
Void
}
```
</td>
</tr>
<tr>
<td>
```aiken
fn weird_maths() -> Bool {
expect 1 == 2
}
```
</td>
<td>
```aiken
fn weird_maths() -> Bool {
expect True = 1 == 2
True
}
```
</td>
</tr>
</tbody>
</table>
The point of those tests is to ensure that blueprints are generated
properly, irrespective of the generated code. It is annoying to
constantly get those test failing every time we introduce an
optimization or something that would slightly change the generated
UPLC.
