These accessors used to be necessary (in addition to `.shape()`) while
the AST nodes were mutable. However, after commit 2bf1b4da that made
AST nodes immutable, there is no technical requirement to keep them
around. Additionally:
- `len(value)` is shorter than `value.width` and works with any `value`
- `value.shape().signed` is longer than `value.signed` but works with
any `value`