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

• sysstr housekeeping:
  • Removed redundant len and reserved sets already performed by prior rawNewStringNoInitcalls.

[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 :)