Add mappers for direct solver

[alexfikl]

This rips out the mappers from #30 for some discussion + tests. The new mappers are

• IntGTermCollector: removes all non-IntG terms in the expression and all non-constant coefficients.
• KernelTransformationRemover: removes transforms from IntG kernels.
• DOFDescriptorReplacer: re-tags an expression.

They're basically used to clean up a given expression for source->proxy or proxy->target evaluation, which

• doesn't need any non-IntG terms, since those are only diagonal operators.
• doesn't need to see any source / target derivatives, but generally also can't use these because we don't want to compute normals and things at the proxy points.

TODO:

• Requires Flatten product in DerivativeBinder.map_product pymbolic#102.

[inducer]

Thanks! Some comments from a quick scroll.

[inducer]

Thanks! A few more things below. LGTM once these are addressed.

[inducer]

Unsubscribing... @-mention or request review once it's ready for a look or needs attention.

[inducer]

This looks good, thanks!