Add num_traits proc_macro helper for explicit import

This allows the user to specify a identifier for the `num_traits` crate
so that the proc_macro can directly depend in it.

This is usefull when reexporting `num-derive` and using `num-trait` as
a transitive import.

Fixes #34.

ci/test_full.sh

@@ -7,3 +7,5 @@ echo Testing num-derive on rustc ${TRAVIS_RUST_VERSION}
 # num-derive should build and test everywhere.
 cargo build --verbose --features="$FEATURES"
 cargo test --verbose --features="$FEATURES"
+cd check; cargo test --verbose check; cd ..
+cd import; cargo test --verbose import; cd ..

import/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "import"
+version = "0.0.1"
+authors = ["jean-airoldie <[email protected]>"]
+edition = "2018"
+publish = false
+
+[dependencies]
+num-derive = { path = "../" }
+num = { version = "0.2", default-features = false }

import/src/lib.rs

@@ -0,0 +1,19 @@
+#[macro_use]
+extern crate num_derive;
+
+#[derive(
+    Debug,
+    Clone,
+    Copy,
+    PartialEq,
+    PartialOrd,
+    ToPrimitive,
+    FromPrimitive,
+)]
+#[num_traits = "num"]
+#[repr(u8)]
+enum Rgb {
+    Red = 0,
+    Green = 1,
+    Black = 2,
+}

import/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}