Ticket 331: Add R6RS base compatibility library

The (scheme r6rs base) library exports the standard identifiers of the base library of R6RS. By my current reckoning, those identifiers are as follows:

- * / + < <= = > >= abs acos and angle append apply asin atan begin boolean? call/cc call-with-current-continuation call-with-values car case cdr ceiling char? char<? char<=? char=? char>? char>=? char->integer complex? cond cons cos define define-syntax denominator dynamic-wind eq? equal? eqv? even? exact exact? exact-integer-sqrt exp expt finite? floor for-each gcd guard if imag-part import inexact inexact? integer? integer->char lambda lcm length let let* let*-values letrec letrec* letrec-syntax let-syntax let-values list list? list->string list->vector list-ref list-tail log magnitude make-polar make-rectangular make-string make-vector map max min nan? negative? not null? number? number->string numerator odd? or pair? positive? procedure? quasiquote quote rational? rationalize real? real-part reverse round set! sin sqrt string string? string<? string<=? string=? string>? string>=? string->list string->number string->symbol string-append string-copy string-for-each string-length string-ref substring symbol? symbol->string tan truncate values vector vector? vector->list vector-fill! vector-for-each vector-length vector-map vector-ref vector-set! zero?

As with any library other than (scheme base), implementations SHOULD (rather than MUST) provide this.