Revision to how charsxp and symsxp are handled, in GC and elsewhere.
Fixes charsxp cache to use ATTRIB field unconditionally, removing other code.
Now does simple check for CHARSXP in DO_CHILDREN in gc.
Changes a symsxp to have a nextsym field, getting rid of the
list fof syms in R_SymbolTable in favour of linking by that field.