add Exports to manage exports handle

A handle from wasmer_instance_exports will have a ownership of all exports, so calling wasmer_exports_destroy will invalidates all handle obtained by wasmer_exports_get. To fix issue Exports is introduces to manage exports handle.

Besides bug fix, I also added omnisharp.json for code formatting. I tried to minimize code changes by code formatter, but there was some inconsistencies in current code so there are some formatting diffs.