Feat: stdlib: adds system.string.setLenUninit

[ZoomRmc]

Adds system.setLenUninit for the string type. Allows setting length without initializing new memory on growth.

• Required for a follow-up to add overload add(a: var string, b: openArray[char]) #15951
• Accompanies ref #19727; implement setLenUninit for seqsv2 #22767 (ref newSeqUninitialized over-constrained, setLenUninitialized missing #19727) but for strings
• Expands stdlib/tstring with tests for setLen and setLenUninit

[ZoomRmc]

systr.setLengthStrUninit fails with mm:refc on Linux.
Seems my naïve approach is wrong.
Can this be made to work without introducing new compiler procs?

[arnetheduck]

the same for seq would be nice, ie strings are pretty niche at the end of the day.
never mind, forgot that it was already fixed :)

[arnetheduck]

ping, would be nice to see this PR merged

[ZoomRmc]

I think I was stuck on not being sure if it's done and both string versions actually work, since some of CI errors at the time were unrelated.

Those string/seq impls is a mess of conditionals and I don't trust locally-run tests.

[ZoomRmc]

Nim/lib/system.nim

Lines 2316 to 2326 in 2e45f61

	when notJSnotNims:
	when defined(nimSeqsV2):
	{.noSideEffect.}:
	let str = unsafeAddr s
	setLengthStrV2Uninit(cast[ptr NimStringV2](str)[], newlen)
	else:
	{.noSideEffect.}:
	when hasAlloc:
	setLengthStrUninit(s, newlen)
	else:
	s.setLen(n)

Adding a hasAlloc check and and else-downgrade to setLen for the old string branch was necessary for this to compile. I'm not sure if it's correct as I'm not sure what constitutes the string type at this point.

[Araq]

Apart from my code review remarks, seems fine.

[ZoomRmc]

So I went with expanding that early return conditional at line 232 a bit and added some internal docs. If it's still not up your alley I can change the code to whatever.

If it's ok as is, do you want me to squash it?

BTW, you can't currently build docs with --docInternal for system due to assertion defect in semtypes:

...assertions.nim(34)       raiseAssert
Error: unhandled exception: semtypes.nim(2471, 7) `c.graph.sysTypes[tySequence] == nil`  [AssertionDefect]```