Fix borrow scope for store in WrappedCallable impl for WasmtimeFn.

This PR fixes the borrow scope of store in the `WrappedCallable` impl of
`WasmTimeFn` such that it does not remain borrowed across the call to
`wasmtime_call_trampoline`. By limiting the scope of the borrow, the
implementation can be reentered if an exported function calls an imported
function, which in turn calls another exported function.

Fixes #365.

wasmtime-api/tests/import_calling_export.wat

@@ -0,0 +1,8 @@
+(module
+  (type $t0 (func))
+  (import "" "imp" (func $.imp (type $t0)))
+  (func $run call $.imp)
+  (func $other)
+  (export "run" (func $run))
+  (export "other" (func $other))
+)