Add RSymbol struct back into RVALUE

[peterzhu2118]

Commit 0ca714f removed RSymbol from RVALUE. This commit adds RSymbol back into RVALUE.

@nobu can you confirm whether the removal was intentional or not?

[nobu]

I think it wasn't used at that time at least.

[peterzhu2118]

Symbol is allocated in the GC as struct RSymbol so I think it should be in the union:

ruby/symbol.c

Line 611 in df7efdc

const VALUE dsym = rb_newobj_of(klass, T_SYMBOL | FL_WB_PROTECTED);

All the current places use the RSYMBOL macro to overcome this issue, but I think since it's allocated in the GC it should be in the union.