Have a logger-like warning precedence concept

[Kon Lovett]

• level is a symbol or #f, indicating lower than the lowest level

(message-levels [levels]) -> level

• parameter that gets/sets the active message levels

• levels is a list of symbol defining the legal set of levels and the order of precedence. () means no levels

(message-level [level]) -> level

• parameter that gets/sets the active message level

• the active message level determines whether a message is processed any further. any message with a level lower than the current level is ignored.

(message level msg . args)

• msg is a string, and args are objects

• a level of #f will never be seen unless () = (message-levels)

(warning msg . args) & (##sys#notice msg . args)

• wrappers around `message'