Optimize internal operations in E-Graphs

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The EGraph type and operations extensively use dictionaries. Consider performance tips from the Julia manual:
https://docs.julialang.org/en/v1/manual/performance-tips/

• Replace unefficient/memory hungry data structures (dictionaries, sets, dynamic arrays) with more efficient and specific (e.g. static arrays, sparse matrices) ones.
• Consider using StaticArrays
• Avoid containers with abstract type parameters
• Annotate values taken from untyped locations
• Write Type Stable Functions
• Avoid Changing Variable Types
• Use Loop Vectorization where possible: when the execution order does not matter.
• Use parallel for loops with @threads or FLoops.jl

From https://dl.acm.org/doi/pdf/10.1145/3434304 :
"egg sorts e-nodes within each e-class ot enable binary search and also maintains a cache mapping function symbols to e-classes that contain e-nodes with that function symbol"