bug(generator): Generator cannot infer `ssz-size` for fixed length bytes.

[itsdevbear]

This is a potential footgun as it will generate a valid type. However this type is treated as a composite.

Recommendation:

• Generator should either: a) error and force the user to supply ssz-size:xx or b) be able to infer the size.

Testcase here: https://github.com/ferranbt/fastssz/pull/154

[ferranbt]

Does #155 fixes the issue?