Add options to control recompilation

[JustusAdam]

It would be super useful if the plugin could specify a recompilation policy.

Possible useful policies I can think if could be

• Always recompile
• Recompile if arguments/configuration changes
• Recompile if plugin defined state changes
• Default rustc behavior

Either of these policies may then also apply to dependencies or the workspace.

A definition of a policy could look something like this

enum RecompilationPolicy<Plugin> {
    /// Recompile every time the tool is run
    Always,
    /// Spiritually the same as `WhenStateChanges(Hash::hash)`
    WhenArgsChange,
    /// Recompile when the state changes. Change here is defined as the
    /// arguments and potentially external state (hence the function pointer to
    /// fetch external state) hashes to a new value.
    /// 
    /// Can also be used to only depend on a subset of the arguments.
    WhenStateChanges(for<'a> fn (&'a Plugin::Args) -> u64),
    Rustc
}

Then RustcPluginArgs could have two fields for the recompilation policy for the workspace and the dependencies respectively, both of type RecompilationPolicy.