fix: call extism_function_free for functions created with extism_function_new

[zshipko]

Fixes #12

Adds calls to free functions associated with a plugin when the plugin gets cleaned up, or if plugin initialization fails.

Not sure if this is the most idiomatic way to handle this, so if there's a better way to do this let me know!

[zshipko]

Actually, I'm don't think this is the right solution after looking at the code a little more. It seems like a Extism::HostEnvironment could be used to make multiple plugins so the functions need to be freed when the Extism::Function or Extism::HostEnvironment get garbage collected.