Group integrals by metadata

Currently UFL groups integrals by `domain`, `integral_type`, `subdomain_id` into `IntegralData` objects.
The form compiler then uses this to generate different kernels.

However, forms with different metadata are grouped, and in FFCx, this can potentially produce an error (see: [#Issue 394](https://github.com/FEniCS/ffcx/issues/394)).

Ideally, we would like to have one kernel by metadata (and possibly a quadrature rule per kernel).
Our proposal is to modify [build_integral_data](https://github.com/FEniCS/ufl/blob/a7426bd8533f2c819f7f164df9c197e277d058c3/ufl/algorithms/domain_analysis.py#L237), to group integrals by metadata.

@wence- Any thoughts on that? Would it be ok for tsfc?