Some minor performance optimisations

[Thorium]

Code clean-up with minor performance optimizations:

• isNull instead of "= null": https://latkin.org/blog/2015/05/18/null-checking-considerations-in-f-its-harder-than-you-think/
• TryGetValue instead of double-lookups of dictionaries
• Types and active patterns that are not containing any data objects can be structs to reduce object capsuling and memory footprint

No functional changes.

[kMutagene]

Thank you for the contribution @Thorium , @omaus can you have a look at this?

[omaus]

Lgtm. Also, learned new things about returning structs in active patterns. 😄