optimizer: eliminate excessive specialization in inlining code

This commit includes several code quality improvements in inlining code:
- eliminate excessive specializations around:
  * `item::Pair{Any, Any}` constructions
  * iterations on `Vector{Pair{Any, Any}}`
- replace `Pair{Any, Any}` with new, more explicit data type `InliningCase`
- remove dead code

Author: aviatesk