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()

crates/api/src/trampoline/func.rs

@@ -2,7 +2,6 @@
 
 use super::create_handle::create_handle;
 use super::trap::{record_api_trap, TrapSink, API_TRAP_CODE};
-use crate::r#ref::HostRef;
 use crate::{Callable, FuncType, Store, Val};
 use anyhow::Result;
 use std::cmp;
@@ -234,7 +233,7 @@ fn make_trampoline(
 pub fn create_handle_with_function(
     ft: &FuncType,
     func: &Rc<dyn Callable + 'static>,
-    store: &HostRef<Store>,
+    store: &Store,
 ) -> Result<InstanceHandle> {
     let sig = ft.get_wasmtime_signature().clone();
 
@@ -270,7 +269,7 @@ pub fn create_handle_with_function(
 
     create_handle(
         module,
-        Some(store.borrow_mut()),
+        Some(store),
         finished_functions,
         Box::new(trampoline_state),
     )

crates/api/src/trampoline/mod.rs

@@ -12,7 +12,6 @@ use self::global::create_global;
 use self::memory::create_handle_with_memory;
 use self::table::create_handle_with_table;
 use super::{Callable, FuncType, GlobalType, MemoryType, Store, TableType, Val};
-use crate::r#ref::HostRef;
 use anyhow::Result;
 use std::rc::Rc;
 
@@ -22,7 +21,7 @@ pub use self::trap::take_api_trap;
 pub fn generate_func_export(
     ft: &FuncType,
     func: &Rc<dyn Callable + 'static>,
-    store: &HostRef<Store>,
+    store: &Store,
 ) -> Result<(wasmtime_runtime::InstanceHandle, wasmtime_runtime::Export)> {
     let mut instance = create_handle_with_function(ft, func, store)?;
     let export = instance.lookup("trampoline").expect("trampoline export");