bc: add strict (#554)

* In yyrule list, strings were not escaped properly
* Use yyclearin() where possible to reset $yychar
* Eliminate #line directives which cause perl debugger to print misleading line numbers
* In grammar rule 84 (expr=IDENT), access $yyvs[$yyvsp] directly instead of accessing a hash inside it which doesn't exist (this is consistent with rules 86,87,etc
* Replace string-eval with block-eval in main()