fixes #25007; implements setLenUninit for refc

[ringabout]

fixes #25007

proc setLengthSeqUninit(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
    compilerRtl.} =

In this added function, only the line zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize) is removed from proc setLengthSeqV2 when enlarging a sequence.

JS and VM versions simply use setLen.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 611b8bb

Hint: mm: orc; opt: speed; options: -d:release
182867 lines; 8.794s; 659.902MiB peakmem

[ZoomRmc]

Looks like when setLengthSeqUninit called for s == nil and newLen != 0 it calls basic newSeq which relies on zeroing newObj for initialization.

There's a separate newSeq implementation for each GC, so it seems like the easiest path is to modify them to conditionally call newObj or newObjNoInit (which is not defined for all the GCs).