Enhancement: Make use of package extensions

Speed up load time of the package by using [package extensions](https://pkgdocs.julialang.org/v1/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions)).

Candidates: `SparseArrays`, `FillArrays`, `PooledArrays`,  `Accessors`

Bonus: do the same in [`JsonGrinder.jl`](https://github.com/CTUAvastLab/JsonGrinder.jl).