perf: decrease parsing related garbage by using more transducers

[thenonameguy]

There are a few places internally that use unnecessary intermediate object allocation, especially operations over maps.
One project which has helped with this problem is https://github.com/cgrand/xforms
It has a syntactically easy abstraction of creating reduce-kv equivalent transducing contexts which avoid intermediate k/v vectors.

[wilkerlucio]

I already do use a lot of transducers in most of the code, did you know specific parts that are being problematic?

[thenonameguy]

Mostly (into {} xform seq) can be optimizied, like:

pathom/src/com/wsscode/pathom/trace.cljc

Line 52 in 8a43e19

(let [leave-items (into {} (comp (filter (comp #(= ::leave %) ::direction))

To not allocate intermediate vectors via juxt, by using (x/into {} (x/for ...) seq), like detailed here:
https://github.com/cgrand/xforms#on-key-value-pairs

[wilkerlucio]

Not sure how much that would improve, and currently I have no time to prioritize this. If you really feel this is a good approach, I could look into a PR with the changes, just can't do it myself.