Explore Immutable Data Structures

[mikong]

Is your feature request related to a problem? Please describe.
Performance. The project currently uses the standard library HashMap. When the project becomes more stable and have a better set of benchmark tests, we should explore alternative data structures that might provide better performance.

Describe the solution you'd like
Use the im crate and compare its performance with the standard library HashMap.

Describe alternatives you've considered
Alternatively, we can make smaller changes first, such as using a faster hashing algorithm for the standard library HashMap. We'll have to consider the security implications of using other hashing functions though.

Additional context
The Feature Request issue template was recently added to this project, and I thought of trying it out with a feature that I've listed in my internal notes for this project.