Implement Policies

[devreal]

Policies are a mechanism to steer the execution of tasks through a single entity controlling different properties. The interface is extensible to allow for the addition of future properties without breaking backwards compatibility. If desired, providing a custom policy allows the compiler to fully inline all calls and thus avoid a bunch of indirect calls through std::function. This is useful for small tasks and allows us to reduce the overhead of querying these properties.

Currently, three properties are supported:

1. Process map (aka keymap): map a key identifying a task to a process for execution. Implemented using the int procmap(const Key&) member.
2. Priority map: map a key identifying a task to a priority. Implemented using the int priomap(const Key&) member.
3. Inline map: map a key identifying a task to a max depth for inlining. Implemented using the int inlinemap(const Key&) member. This has yet to be implemented in the backends.

Policies can be specified in two ways:

1. Using make_policy() to work with make_tt. Example:

return ttg::make_tt([](){ /* task function */ }, edges(...), edges(...), 
                ttg::make_policy([](const Key& key){ return key % world.size(); }, 
                                 [](const Key& key){ return key.x; /* priority */}));

Functions are defaulted to make sure that all necessary properties are implemented. A function that is not provided can later be set dynamically on the tt (using tt->set_inlinemap([](const Key&){ return 0; }) in the example above).

2. Using a custom policy class:

struct MyPolicy : public ttg::TTPolicyBase {
  ttg::World world;
  MyPolicy(ttg::World world)
  : world(world), TTPolicyBase()
  { }
  int procmap(const Key& key) {
    return key % world.size();
  }
};

struct MyTT : ttg::TT<..., MyPolicy>
{
  ...
}

The procmap member function will shadow the procmap member of ttg::TTPolicyBase. Since the TT is templated on the policy, the procmap of MyPolicy is used. There is no need for virtual functions here. Similarly to the above, properties not specified in MyPolicy are defaulted to a std::function and can be set dynamically.

This PR is based on #217 to be able to default the policy type of TT so the diff is bigger than it will be if #217 gets merged.

[devreal]

Superseded bu #226, closing