This patch allows safer (note: safer, not safe) embedding in a plugin
environment where multiple instances of the engine could be needed.
Original code initializes and tears down the full runtime during
wasm_engine_new() and wasm_engine_delete() respectively. After this
update the C API implementation keeps track of engine instances count
and inits/deinits the runtime only when needed.
This allows for example to call wasm_engine_new() twice and then call
wasm_engine_delete() once without rendering the first engine instance
invalid.
f8ee05db4b