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Implicit return values (`null`), closes #217

This PR makes it possible to skip specifying a
return value, a common scenario when we want to exit,
for example, an IF statement:

```
if something {
  return;
}
```

Note that a value-less return statement needs to end with
a semicolon, else we would need substantial changes to our
lexer in order to carry forward new line breaks. Currently,
line breaks are not passed onto the parser, so the parser
wouldn't know whether to trigger a default return value
based on a line break:

```
if something {
  return # the parser would try to go ahead and parse the '{' as a return value
}
```

This also means there's a tricky behavior (documented in the docs),
as if the user forgets to include a semicolon and the next line contains
a valid expression, the expression will be used as return value:

```
if something {
  return
  1
} # the return value is 1
```

All considered, I think these trade-offs are ok, and rather than
undergoing an extensive refactoring of lexer/parser I would throw
this feature out there and then figure out if the community is too
annoyed by these couple of quirks...

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