Unit and optional attributes

[Dinistro]

For some operations/types it would be nice to have optional attributes or unit attributes/parameters.

For example, MLIR's memref.global has a unit attribute constant which, when present, indicates that this global is constant.
Another one are IntegerTypes, which can have a "signedness" parameter which indicates that the type is a signed integer.

One could also add boolean attributes but this might clutter the output a lot.

[Dinistro]

Optional attributes are done, but support for unit attributes will still be helpful.
In devito, they add a lot of meta information to operations, like affine, parallel, skewable, etc. to indicate certain properties of a operation. Currently, these properties are added to a properties ArrayAttr which contains StringAttr's. While this works, it is very cumbersome to use.

[Dinistro]

Fixed by #161