Add a wasmtime::Linker type (#1384)

* Add a `wasmtime::Linker` type

This commit adds a new type to the `wasmtime` crate, a `Linker`. This
linker is intended to vastly simplify calling `Instance::new` by easily
performing name resolution and incrementally defining state over time.
The goal here is to start down a path of making linking wasm modules in
`wasmtime` a first-class and ergonomic operation. This is highly likely
to evolve over time and get tweaked through releases as we iterate
towards a design well-suited for `wasmtime`, but this is intended to at
least be the initial foundation for such functionality.

This commit additionally also adds a C API for the linker and switches
the existing linking examples to using this linker in both Rust and C.

One piece of future work I'd like to tackle next is to integrate WASI
into the `wasmtime` crate in a more first-class manner. This [`Linker`]
type provides a great location to hook into the instantiation process to
easily instantiate modules with WASI imports. That's a relatively large
refactoring for now though and I figured it'd be best left for a
different time.

Closes #727

crates/c-api/src/wasi.rs

@@ -168,7 +168,7 @@ pub unsafe extern "C" fn wasi_config_preopen_dir(
 
 #[repr(C)]
 pub struct wasi_instance_t {
-    wasi: Wasi,
+    pub wasi: Wasi,
     export_cache: HashMap<String, Box<wasm_extern_t>>,
 }