map needs clear

[jesse99]

map needs a clear or remove_all method. This is especially important because I don't think there is a good work around. Obvious candidates are 1) remove each key individually which is awful performance wise 2) use a mutable field or slot and assign a new map which, in my case at least, caused a cascade of illegal borrows.

[catamorphism]

Changed wishlist label to enhancement -- wishlist is for the really ambitious stuff, and this shouldn't be too hard :-)

[gwillen]

I believe this is now resolved. (I can't close it.)

[catamorphism]

Thanks @gwillen !