Instead of supplying an extern fn, a simpler native rust fn is supplied
and the BlockArguments trait is used to get an extern fn that wraps it.

Previous code was just assigning to an uninitialzed field, so the
compiler was dropping it; by changing this to a ptr::write the existing
value is no longer dropped.

The old behavior can be replicated by creating a custom closure with a
context and function.

Disabled the msg_send! and method! macros.

This reverts commit 3c6d05c.

Fixes #8.

(cherry picked from commit c93b6a1)

The following warnings appear:

```
src/lib.rs:73:5: 73:68 warning: the trait `core::marker::Sized` is not implemented for the type `Self` [E0277]
src/lib.rs:73     unsafe fn call_block<R>(self, block: *mut Block<Self, R>) -> R;
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:73:5: 73:68 help: run `rustc --explain E0277` to see a detailed explanation
src/lib.rs:73:5: 73:68 note: `Self` does not have a constant size known at compile-time
src/lib.rs:73     unsafe fn call_block<R>(self, block: *mut Block<Self, R>) -> R;
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:73:5: 73:68 note: this warning results from recent bug fixes and clarifications; it will become a HARD ERROR in the next release. See RFC 1214 for details.
src/lib.rs:73     unsafe fn call_block<R>(self, block: *mut Block<Self, R>) -> R;
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:73:5: 73:68 note: required by `Block`
src/lib.rs:73     unsafe fn call_block<R>(self, block: *mut Block<Self, R>) -> R;
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:187:5: 187:71 warning: the trait `core::marker::Sized` is not implemented for the type `Self` [E0277]
src/lib.rs:187     fn into_concrete_block(self) -> ConcreteBlock<A, Self::Ret, Self>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:187:5: 187:71 help: run `rustc --explain E0277` to see a detailed explanation
src/lib.rs:187:5: 187:71 note: `Self` does not have a constant size known at compile-time
src/lib.rs:187     fn into_concrete_block(self) -> ConcreteBlock<A, Self::Ret, Self>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:187:5: 187:71 note: this warning results from recent bug fixes and clarifications; it will become a HARD ERROR in the next release. See RFC 1214 for details.
src/lib.rs:187     fn into_concrete_block(self) -> ConcreteBlock<A, Self::Ret, Self>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:187:5: 187:71 note: required by `ConcreteBlock`
src/lib.rs:187     fn into_concrete_block(self) -> ConcreteBlock<A, Self::Ret, Self>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

...because of:

rust-lang/rfcs#1214

This warning starting appearing from rust-lang/rust#26583.