_free function not exported in wasm module generated by plugin-wasm

[chrispcampbell]

The WasmModule interface expects the _free function to be exported (for the purposes of disposing a WasmBuffer). Up until now, the WasmBuffer.dispose method wasn't called as part of other constructs, but when I started fixing that (for #471), the dispose call would throw an error because _free is not available.

We need to update EXPORTED_FUNCTIONS in plugin-wasm to include _free and update related documentation.