This mimics how std does it, e.g. std::vec::Vec. This avoids potential
problems down the road, where adding more types might clutter the
API interface and generated documentation.
SparsityPattern's constructor now returns a fine-grained error
enum that enumerates possible errors. We use this to build a more
user-friendly error when constructing CSR matrices.
We also overhauled the main SparseFormatError error type by
making it a struct containing a *Kind type and an underlying error
that contains the message.
We can easily create the CSR and CSC constructors by using the
SparsityPattern constructors. However, one lingering problem
is giving meaningful error messages. When forwarding error messages
from the SparsityPattern constructor, the error messages must
be written in terms of "major" or "minor" dimensions, and using
general terms like "lanes", instead of "rows" and "columns".
When forwarding these messages up to CSR or CSC constructors,
they are not directly meaningful to an end user. We should find
a better solution to this problem, so that end users get
more meaningful messages.
