auto merge of #9250 : erickt/rust/num, r=erickt

This PR solves one of the pain points with c-style enums. Simplifies writing a fn to convert from an int/uint to an enum. It does this through a `#[deriving(FromPrimitive)]` syntax extension.

Before this is committed though, we need to discuss if `ToPrimitive`/`FromPrimitive` has the right design (cc #4819). I've changed all the `.to_int()` and `from_int()` style functions to return `Option<int>` so we can handle partial functions. For this PR though only enums and `extra::num::bigint::*` take advantage of returning None for unrepresentable values. In the long run it'd be better if `i64.to_i8()` returned `None` if the value was too large, but I'll save this for a future PR.

Closes #3868.

Author: bors

doc/rust.md

@@ -1775,6 +1775,7 @@ Supported traits for `deriving` are:
   `obj.to_str()` has similar output as `fmt!("%?", obj)`, but it differs in that
   each constituent field of the type must also implement `ToStr` and will have
   `field.to_str()` invoked to build up the result.
+* `FromPrimitive`, to create an instance from a numeric primitve.
 
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