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.

test-all.sh

@@ -52,6 +52,7 @@ banner "Rust unit tests"
 #RUST_BACKTRACE=1 cargo test --all
 RUST_BACKTRACE=1 cargo test \
   --package wasmtime \
+  --package wasmtime-api \
   --package wasmtime-wasi \
   --package wasmtime-wast \
   --package wasmtime-debug \