Redesign From derive macro (#241)

Replace `#[from(types(i32, "&str"))]` with `#[from(i32, &str)]` and add
support for deriving multi-field structs and enum variants with
`#[from((<tuple>), (<tuple>), ...)]`.

Author: ilslv

CHANGELOG.md

@@ -31,6 +31,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - The `From` derive doesn't derive `From<()>` for enum variants without any
   fields anymore. This feature was removed because it was considered useless in
   practice.
+- The `From` derive now uses `#[from(<types>)]` instead of `#[from(types(<types>))]`
+  and ignores field type itself.
 
 ### Added
 

Cargo.toml

@@ -33,6 +33,7 @@ rustc_version = { version = "0.4", optional = true }
 
 [dev-dependencies]
 rustversion = "1.0"
+static_assertions = "1.1"
 trybuild = "1.0.56"
 
 [badges]
@@ -229,7 +230,7 @@ required-features = ["try_unwrap"]
 [[test]]
 name = "compile_fail"
 path = "tests/compile_fail/mod.rs"
-required-features = ["debug", "display"]
+required-features = ["debug", "display", "from"]
 
 [[test]]
 name = "no_std"