Some comments about how to improve value numbering with symmetries.
Document representation dict value types with some internal comments.
Move parts of uflacs.ffc.backend into various files.
Delete dummy code.
Some minor refactoring and some minor memory saving optimizations.
Implement value numbering for modified terminals with symmetries from
elements and multiple derivatives.
some comments
Remove erroneous assertion for x in dS context.
Fixes for making interval integrals work.