Builtin deriving-generated attributes should be named better

[Manishearth]

Currently, these show up as __arg_0, etc in docs:

We should provide the ability for builtin derives to specify a string for each argument name, e.g. the args vector here (usage) would be a vector of tuples.

To fix this we also need to generate identifiers hygenically. Currently we generate __arg_0 so that it does not clash with other constants, but instead we should be doing something like ident_of("argumentname").gensym() which we then pass around.

(Ideally, we'd do the same for the other double underscored idents in the deriving code)

@zofrex is working on this

[nsmaciej]

This looks like it can be closed.