hdl.ast: add an explicit Shape class, included in prelude.

hdl.ast: add an explicit Shape class, included in prelude.

Shapes have long been a part of nMigen, but represented using tuples.
This commit adds a Shape class (using namedtuple for backwards
compatibility), and accepts anything castable to Shape (including
enums, ranges, etc) anywhere a tuple was accepted previously.

In addition, `signed(n)` and `unsigned(n)` are added as aliases for
`Shape(n, signed=True)` and `Shape(n, signed=False)`, transforming
code such as `Signal((8, True))` to `Signal(signed(8))`.
These aliases are also included in prelude.

Preparation for #225.