Companion to readRawData whose pointer stays valid across moves/copies
of the string. Under --strings:sso it promotes a small inline string to
its heap representation; under refc/v2 the data is already heap-resident
so it aliases readRawData.
Uniform `var string` signature on every backend so code can prepare for
--strings:sso without `when declared`.
Adds `system.setLenUninit` for the `string` type. Allows setting length
without initializing new memory on growth.
- Required for a follow-up to #15951
- Accompanies #22767 (ref #19727) but for strings
- Expands `stdlib/tstring` with tests for `setLen` and `setLenUninit`
---------
Co-authored-by: Andreas Rumpf <araq4k@proton.me>
