Added relax strategy parameter

[zesterer]

This currently has issues due to rust-lang/rust#50822. Event fairly mundane examples will not compile because the compiler does not infer the default parameter without a type annotation, and type namespace paths are not type annotations.

The following example...

let mylock = spin::Mutex::new(0);

...fails with...

error[E0282]: type annotations needed for `spin::Mutex<i32, R>`
 --> test.rs:1:14
  |
1 | let mylock = spin::Mutex::new(0);
  |     ------   ^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `R`
  |     |
  |     consider giving `mylock` the explicit type `spin::Mutex<i32, R>`, where the type parameter `R` is specified

error: aborting due to previous error

I'm undecided about what approach to use to solve this. Perhaps we should use a series of type aliases within the crate to specify a default Spin relax strategy, thereby avoiding major breakage for most users but allowing advanced users to opt-in, albeit through a more unconventional way of accessing the types?

[zesterer]

I've updated the branch to add the aforementioned type aliases. This should avoid compiler errors for ordinary users. Advanced users can switch relax strategy by using the fully-qualified path for a lock.