Fix excessive memory usage for Rust 1.76

[atuchin-m]

For cr121 we started to use 1.76 rust with a in-place iteration issue that results in excessive memory consumption.
rustc 1.76.0-dev (df0295f07175acc7325ce3ca4152eb05752af1f2-3-llvmorg-18-init-12938-geb1d5065 chromium)

This PR fixes the most critical code path that reduces the memory to about the original size.

[antonok-edm]

@atuchin-m does it make sense to move it to line 807, next to the other shrink_to_fit?

[atuchin-m]

@antonok-edm we need to call shrink_to_fit() for each key-value we add to the map.

And we can't do it after. In that case the map takes already much more memory that we need to. That affect peak memory usage and create extra work to alloc and to free after.

With local allocs/deallocs I suppose PartAlloc cache could bear them without much using OS-level alloc.