Support Func imports with zero shims (#839)

* Move `Func` to its own file

* Support `Func` imports with zero shims

This commit extends the `Func` type in the `wasmtime` crate with static
`wrap*` constructors. The goal of these constructors is to create a
`Func` type which has zero shims associated with it, creating as small
of a layer as possible between wasm code and calling imported Rust code.

This is achieved by creating an `extern "C"` shim function which matches
the ABI of what Cranelift will generate, and then the host function is
passed directly into an `InstanceHandle` to get called later. This also
enables enough inlining opportunities that LLVM will be able to see all
functions and inline everything to the point where your function is
called immediately from wasm, no questions asked.

crates/runtime/src/lib.rs

@@ -43,6 +43,7 @@ pub use crate::instance::{InstanceHandle, InstantiationError, LinkError};
 pub use crate::jit_int::GdbJitImageRegistration;
 pub use crate::mmap::Mmap;
 pub use crate::sig_registry::SignatureRegistry;
+pub use crate::trap_registry::TrapDescription;
 pub use crate::trap_registry::{get_mut_trap_registry, get_trap_registry, TrapRegistrationGuard};
 pub use crate::traphandlers::resume_panic;
 pub use crate::traphandlers::{raise_user_trap, wasmtime_call, wasmtime_call_trampoline, Trap};