feat!: Add SBO support for lambdas and functors delegates

This commit adds 'Small Buffer Optimization' to functor and lambda functions with
small footprint, i.e. the same size as 'void *' or less, and will therefore mimic the
`std::function` behavior for these types, instead of being a reference
to them as before.

This means that the following operations are now allowed to delegate
with deferred/out-of-scope calls:

```
etl::delegate<void()> Class::method() {
  etl::delegate<void()> d {[this](){ this->do_something(); }};
  return d;
}
```

A caveat is that the old behavior where everything became a reference instead
of an object is changed and the user must now use a `reference_wrapper`
to gain the same behavior, i.e. using `etl::ref` or `etl::cref`:

```
  auto f = [&](){ /* ... */ };
  etl::delegate<void()> d { etl::ref(d) };
  d();
```

This change of behavior also matches the C++ Core Guidelines for how
to pass parameters
[https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#f15-prefer-simple-and-conventional-ways-of-passing-information]

BREAKING CHANGE: functors and lambda capture will not be references to any
longer.