+* Improved the speed of subs(), normal(), to_rational() and to_polynomial()
+ by the use of maps instead of lists. The old forms
+ subs(const lst & ls, const lst & lr, unsigned options)
+ to_rational/to_polynomial(lst & repl)
+ are still available for compatibility, but using the new forms
+ subs(const exmap & m, unsigned options)
+ to_rational/to_polynomial(exmap & repl)
+ is more efficient, especially when the number of replacements is large.