Implement BiCGSTAB

[jlogan03]

Some thoughts about this implementation -

Pros

• It runs and is even fairly correct and a bit documented
• Implemented on f32 and f64 with generic indices

Cons

• Doesn't represent any particular reusable abstract pattern of iterative solver interface, and only makes a little progress toward the goals of #339
• Solver call signature requires the turbof***
  • I haven't found any way around this - even when it's obvious from the inputs and outputs which variant should be selected, it still fails to select the correct variant without explicit type annotation
• Macros make things worse for the users in terms of readability, understanding errors, etc
• Not fully generic on value type
• Not generic on sparse vs. dense RHS and initial guess
• Can't be implemented directly on Complex types
• Allocates then drops several vectors on each iteration which have the same sparsity at every iteration and could reuse the same memory if we had a mechanism for this
  • In practice, this isn't as bad as it sounds since the first few iterations will clear out a correctly sized region of heap s.t. the subsequent allocations are cheap to perform... as long as there is not much else happening on the machine

I spent a solid 8 hours attempting to make a version that is generic over the value type. This did not go well.

Some of the implementations of the underlying matrix-vector and vector-scalar operations are not fully generic, so the trait bounds on the impl end up being more than 10 lines long and incredibly unclear as to which types will and will not be compatible, and the type system ultimately fails on overflow due to inferring generic outputs across iterations in the solver loop.

This could all be fixed to some extent, but would require quite a bit of work to go back into the core of the crate and make a single definition of what trait bounds constitute a mathematical field over which we can do linear algebra, then propagate that definition to every struct and impl in the crate, which I believe is out of scope for this PR.

[jlogan03]

@mulimoen I like the approx usage for testing, but had to roll it back because we don't keep approx as a default feature so it crashes the tests. I'd be happy to bring it back if we want to update either default features to include approx, or to include the approx feature for testing

[mulimoen]

I don't feel strongly about approx, bit of a pain to enable it for doctests. A bit of resolver history: The resolver previously merged features across build types, so having approx on during tests would enable it also for regular builds. Since we are using resolver v2 we can now use it for tests only