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/runtime/src/lib.rs

@@ -45,9 +45,7 @@ pub use crate::sig_registry::SignatureRegistry;
 pub use crate::table::Table;
 pub use crate::trap_registry::{TrapDescription, TrapRegistration, TrapRegistry};
 pub use crate::traphandlers::resume_panic;
-pub use crate::traphandlers::{
-    catch_traps, raise_lib_trap, raise_user_trap, wasmtime_call_trampoline, Trap,
-};
+pub use crate::traphandlers::{catch_traps, raise_lib_trap, raise_user_trap, Trap};
 pub use crate::vmcontext::{
     VMCallerCheckedAnyfunc, VMContext, VMFunctionBody, VMFunctionImport, VMGlobalDefinition,
     VMGlobalImport, VMInvokeArgument, VMMemoryDefinition, VMMemoryImport, VMSharedSignatureIndex,