Remove the need for HostRef<Store> (#771)

* Remove the need for `HostRef<Store>`

This commit goes through the public API of the `wasmtime` crate and
removes the need for `HostRef<Store>`, as discussed in #708. This commit
is accompanied with a few changes:

* The `Store` type now also implements `Default`, creating a new
  `Engine` with default settings and returning that.

* The `Store` type now implements `Clone`, and is documented as being a
  "cheap clone" aka being reference counted. As before there is no
  supported way to create a deep clone of a `Store`.

* All APIs take/return `&Store` or `Store` instead of `HostRef<Store>`,
  and `HostRef<T>` is left as purely a detail of the C API.

* The `global_exports` function is tagged as `#[doc(hidden)]` for now
  while we await its removal.

* The `Store` type is not yet `Send` nor `Sync` due to the usage of
  `global_exports`, but it is intended to become so eventually.

* Touch up comments on some examples

* Run rustfmt

src/commands/wast.rs

@@ -4,7 +4,7 @@ use crate::{init_file_per_thread_logger, pick_compilation_strategy, CommonOption
 use anyhow::{bail, Context as _, Result};
 use std::{fmt::Write, path::PathBuf};
 use structopt::{clap::AppSettings, StructOpt};
-use wasmtime::{Config, Engine, HostRef, Store};
+use wasmtime::{Config, Engine, Store};
 use wasmtime_environ::cache_init;
 use wasmtime_wast::WastContext;
 
@@ -65,7 +65,7 @@ impl WastCommand {
             config.cranelift_opt_level(wasmtime::OptLevel::Speed);
         }
 
-        let store = HostRef::new(Store::new(&Engine::new(&config)));
+        let store = Store::new(&Engine::new(&config));
         let mut wast_context = WastContext::new(store);
 
         wast_context