hdl.ast: avoid unnecessary sign padding in ArrayProxy.

hdl.ast: avoid unnecessary sign padding in ArrayProxy.

Before this commit, ArrayProxy would add sign padding (an extra bit)
a homogeneous array of signed values, or an array where all unsigned
values are smaller than the largest signed one. After this commit,
ArrayProxy would only add padding in arrays with mixed signedness
where all signed values are smaller or equal in size to the largest
unsigned value.

Fixes #476.

Co-authored-by: Pepijn de Vos <pepijndevos@gmail.com>