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/src/trampoline/create_handle.rs

@@ -3,7 +3,7 @@
 use crate::runtime::Store;
 use anyhow::Result;
 use std::any::Any;
-use std::cell::{RefCell, RefMut};
+use std::cell::RefCell;
 use std::collections::{HashMap, HashSet};
 use std::rc::Rc;
 use wasmtime_environ::entity::PrimaryMap;
@@ -13,7 +13,7 @@ use wasmtime_runtime::{Imports, InstanceHandle, VMFunctionBody};
 
 pub(crate) fn create_handle(
     module: Module,
-    signature_registry: Option<RefMut<Store>>,
+    signature_registry: Option<&Store>,
     finished_functions: PrimaryMap<DefinedFuncIndex, *const VMFunctionBody>,
     state: Box<dyn Any>,
 ) -> Result<InstanceHandle> {
@@ -31,7 +31,7 @@ pub(crate) fn create_handle(
 
     // Compute indices into the shared signature table.
     let signatures = signature_registry
-        .map(|mut signature_registry| {
+        .map(|signature_registry| {
             module
                 .signatures
                 .values()