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

crates/api/examples/hello.rs

@@ -1,6 +1,6 @@
 //! Translation of hello example
 
-use anyhow::{ensure, format_err, Context as _, Result};
+use anyhow::{ensure, Context as _, Result};
 use std::rc::Rc;
 use wasmtime::*;
 
@@ -15,11 +15,11 @@ impl Callable for HelloCallback {
 }
 
 fn main() -> Result<()> {
-    // Configure the initial compilation environment, creating more global
-    // structures such as an `Engine` and a `Store`.
+    // Configure the initial compilation environment, creating the global
+    // `Store` structure. Note that you can also tweak configuration settings
+    // with a `Config` and an `Engine` if desired.
     println!("Initializing...");
-    let engine = Engine::default();
-    let store = HostRef::new(Store::new(&engine));
+    let store = Store::default();
 
     // Next upload the `*.wasm` binary file, which in this case we're going to
     // be parsing an inline text format into a binary.
@@ -59,10 +59,7 @@ fn main() -> Result<()> {
 
     // And last but not least we can call it!
     println!("Calling export...");
-    run_func
-        .borrow()
-        .call(&[])
-        .map_err(|e| format_err!("> Error calling function: {:?}", e))?;
+    run_func.borrow().call(&[])?;
 
     println!("Done.");
     Ok(())