Speed up the WCGI path

[misalcedo]

Some ideas to test out:

• [x] Add a mapping of file extension to special handlers using dynamic dispatch in order to have an instance of a handler that can cache state and reuse the engine.
• [x] Maintain a cached copy of a pre instantiated module based on the last modified time stamp in the file metadata.
• [x] Make the module cache optional and configured via a CLI flag.
• [x] Add a preload flag to serve so the server will pre-compile all WCGI scripts.
• [x] Watch the cgi bin directory for any file changes related to wcgi files.

[misalcedo]

I updated the server to generate the wasm and process scripts. Now wasm and process have a single instance per server stored on the server context.

[misalcedo]

Re-using the engine had no discernible difference in performance yet.

[misalcedo]

The notify crate could be used to listen for deletes. Alternatively, I could use glob to list all of the paths that still exist and then delete any paths that no longer exist. Also, could list the cache keys and check if the file it points to still exists; deleting the entry if not.

[misalcedo]

Instead of watching the directory, I will just scan periodically.