Refactor the internals of Func to remove layers of indirection (#1363)

* Remove `WrappedCallable` indirection

At this point `Func` has evolved quite a bit since inception and the
`WrappedCallable` trait I don't believe is needed any longer. This
should help clean up a few entry points by having fewer traits in play.

* Remove the `Callable` trait

This commit removes the `wasmtime::Callable` trait, changing the
signature of `Func::new` to take an appropriately typed `Fn`.
Additionally the function now always takes `&Caller` like `Func::wrap`
optionally can, to empower `Func::new` to have the same capabilities of
`Func::wrap`.

* Add a test for an already-fixed issue

Closes #849

* rustfmt

* Update more locations for `Callable`

* rustfmt

* Remove a stray leading borrow

* Review feedback

* Remove unneeded `wasmtime_call_trampoline` shim

crates/api/src/lib.rs

@@ -8,7 +8,6 @@
 
 #![deny(missing_docs, intra_doc_link_resolution_failure)]
 
-mod callable;
 mod externals;
 mod frame_info;
 mod func;
@@ -21,7 +20,6 @@ mod trap;
 mod types;
 mod values;
 
-pub use crate::callable::Callable;
 pub use crate::externals::*;
 pub use crate::frame_info::FrameInfo;
 pub use crate::func::*;