[Rd] GC: improving the marking performance for STRSXPs

Hi all, in GC (in src/main/memory.c), FORWARD_CHILDREN() (called by PROCESS_NODES()) treats STRSXPs just like VECSXPs, i.e. it calls FORWARD_NODE() for all its children. I claim that this is unnecessarily inefficient since the children of a STRSXP can legitimately only be (atomic) CHARSXPs and

[Rd] GC: parallelizing the CHARSXP cache maintenance

Hi all, As part of RunGenCollect() (in src/main/memory.c), some maintenance on the CHARSXP cache is done, namely unmarked nodes/CHARSXPs are removed from the hash chains. This requires always touching all CHARSXP in the cache, irrespective of the number of generations which were just garbage co