add a test for symbol#succ
add a simple symbol#succ method
add symbol#next as an ersatz alias to #succ

per Alex on IRC, we can't use alias because w_symbol on
space becomes the symbol class, but this code is run
during the construction of the symbol class
proper comment formatting on symbol#next
test symbol#succ with symbol_w instead of str_w and isinstance
Merge pull request #348 from llimllib/symbol_succ

Add Symbol#succ