update attr macro to use argument names where trivial

[cameron1024]

Closes #592

Note for reviewers: 1750 lines of diff comes from the inclusion of a new snapshot file. It's otherwise quite a small PR.

The generated struct will now use the name of an argument if the pattern of the argument is an identifier. For example, given this failing test:

#[property_test] fn test_name(x: i32, #[strategy = "[0-9]*"] s: String) { assert_eq!(x.to_string(), s); }

We now get an error like:

Test failed: assertion `left == right` failed left: "0" right: "1". minimal failing input: TestNameArgs = TestNameArgs { x: 0, s: "1", } 

There are two possible improvements (that I can think of off the top of my head right now) here:

• we could extend this to cover any pattern that binds only a
  single parameter, for example, we could capture this argument as s:

#[property_test] fn foo((Ok(s) | Err(s)): Result<i32, i32>) {}

• we could do some sort of name mangling for more common pattern types, for
  example (a, b): (i32, i32) could correspond to a struct field
  tuple_of_a_b: (i32, i32).

Of these, the second option is a lot easier to implement, though it could get nasty with nested patterns (e.g. tuple_of_tuple_of_tuple_of...).

For now I'm going to get this minimal implementation in though.

[rexmas]

awesome!